Edit: As of 3.1 this is now a dead spec. There is no possible way to salvage it or play it in any form. It is over. Fini. Kaput. Hope thats clear enough. I'll leave the rest of the post for reference, and there is still some useful info in terms of macros and what not.
ShadowFrost is currently the most popular spec for DKs in every bracket. Simply explained, ShadowFrost uses Unholy and Frost talents combined to maximize the damage of Icy Touch.
This allows DKs to do significant and sustained ranged damage with Icy Touch as their primary damage attack, and using Plague Strike and Blood Boil as utility.
The Spec
While there are a variety of small changes that can be made, the basic spec is:
0/21/50
Another less popular spec gets Bladed Armor instead of Killing Machine:
10/11/50
How It Works
The basic premise of the spec is you get the Frost talents which have the greatest effect on Icy Touch: Improved Icy Touch, Glacier Rot, Black Ice, Killing Machine and Deathchill.
Combined with the Frost talents are Unholy talents which further affect Icy Touch: Impurity, Ebon Plaguebringer and Rage of Rivendare (and to a lesser extent Bone Shield and Desecration).
Together, these talents have incredible synergy and give Icy Touch damage approximately equivalent to Scourge Strike and guranteed crits through Killing Machine.
Gearing
Sigil of the Frozen Conscience - This is the bread and butter of the ShadowFrost spec. It increases Icy Touch by 203 damage, but this is affected by debuffs on the mob (Glacier Rot, Ebon Plague, Rage of Rivendare) giving a much larger net effect than the listed 203 damage.
This Sigil is obtained from Venture bay for 30 Venture Tokens. All told, this will take you about half an hour to get.
As to stats, first and foremost, you need to get
5% melee hit or 165 hit rating to ensure your special attacks and white hits do not miss on most targets (some talents will still cause occasional misses, such as Arctic Winds from mages).
Next you will need
4% spell hit. You get 3% from the Virulence talent, so you only need 1% additional spell hit (which you will get easily from capping melee hit). Certain racials cause you to have 2% additional spell miss chance, so to offset this, you will need 6% total spell hit (or Virulence + 3% from gear). Typically this will be completely taken care of by getting 5% melee hit.
You will also want at least
75 spell penetration. You can get this through the cloak enchant and 2 stormy sky sapphires. This will offset most passive resist talents (MotW, Frost Aura) and will lower the chance your spells will be resisted by mages or paladins using auras. Some DKs prefer to get a full 130 spell penetration which will completely negate all resist talents/abilities except Acclimation. Most find that 75 is enough.
Once these stats are taken care of, hit becomes nearly worthless and Strength should be your primary stat choice, followed by attack power and crit.
Glyphs
Icy Touch - With a spec that spams Icy Touch, 10 extra RP per IT cast can be very beneficial in giving extra Death Coils and keeping Gargoyle active. This sigil also allows the player to instantly generate 20 RP, allowing them to use IBF fortitude at the very start of a fight. Many players consider this glyph mandatory.
Blood Boil - Since your primary attack uses a frost rune and applies 2 diseases, this glyph allows you to snare multiple enemies at range, while conserving frost runes for Icy Touch. Combined with the Reaping talent, you also get a Death rune in the deal, allowing even more IT spam. Like the IT glyph, this one is generally considered mandatory.
Ghoul - This is a highly useful glyph, at once giving your ghoul more survivability and more damage. Combined with the Ravenous Dead talent, this will put your ghoul over 18k hp in many gear setups. This gives your healer a better chance to heal it if its being targeted, as well as increases the damage of the ghoul's Explode. Some players choose not to get this glyph.
Plague Strike - Because this spec involves spamming IT for nearly as much damage as a standard Scourge Strike, many players choose not to get Scourge Strike at all. In this case, all Unholy runes must be spent on Plague Strike. With the Desecration talent, along with the hot removal affect of PS and the damage increaes from the Outbreak talent, this glyph provides additional damage, but is far less popular than other glyphs.
Runeforging
Fallen Crusader - As with all specs, Rune of the Fallen Crusader remains a powerful choice for ShadowFrost. It scales with your gear and provides a boost to Gargoyle and melee attacks, along with your Icy Touch and Death Coil. This is the most popular Runeforge by far.
Cinderglacier - This is the alternative choice for runeforging. While it does not provide as long of a boost as a FC proc, it does provide a greater benefit to IT and Death Coil than FC does and thus can be considered more bursty than FC is.
Presences
As ShadowFrost, it is often difficult to know which presence to use and good players disagree over what the "default" presence (or the one they use the most) should be.
Blood Presence offers the hardest hits and highest sustained damage. If your spec does not take Unholy Aura then you wont have a speed bonus in this presence and may want to avoid it. This presence is typically used in games which which take longer (for instance if you 2v2 with a healer and run into another dps/healer comp).
Unholy Presence offers the highest burst damage at the cost of some sustained damage. This presence is most useful in situations where you wont be able to spend a lot of time on a target before the escape (for instance a paladin kiting away from you with freedom or if a rogue is on you stunning frequently) or when you need to burst. Empower Rune Weapon is much stronger in this presence because the shorter GCD allows you to cycle through runes faster. If you dont have Unholy Aura, this presence should probably be your default.
Frost Presence offers by far the best mitigation. If your healer needs to drink or you expect a big burst (e.g. the other team bloodlusts) it is crucial to be able to switch to Frost on short notice to mitigate damage.
Playstyle
When you actually get into arena, you will be dealing primary damage with Icy Touch, while snaring targets with Desecration and Blood Boil. Spreading diseases with Pestilence is important to do better overall damage, make dispelling diseases harder and allow Blood Boil snares on multiple targets.
Deathchill will not be consumed if Killing Machine is active, so don't be afraid to pop it any time during a burst attempt. If KM procs then you will get 2 crit ITs in a row.
Corpse Explosion can be a very effective nuke. With Ebon Plague on the target, Glyph of the Ghoul and Ravenous Dead, this ability, when used on your living ghoul, can hit (NOT crit) for 25% of your ghoul's health (+13% from Ebon Plague) which will typically be 5k or more. The downside is the target must not be moving, so its best used on a target you have stunned or otherwise immobilized. Combine this with Deathchilled IT for huge combos.
Macros
Strangulate
Will Strangulate focus if one is set or your current target if no one focus is set or focus is dead, and cast Deathchill.
/cast Deathchill
/cast [target=focus,exists] Strangulate; Strangulate
Corpse Explosion
Will cast CE on your Ghoul.
/target pet
/cast Corpse Explosion
/targetlasttarget
Gnaw/Leap Focus
Will cause your Ghoul to Gnaw your focus target or leap if he is out of range.
/cast [pet][target=focus] Gnaw
/cast [pet][target=focus] Leap
Presence Swap
Will allow you to switch back and forth between Unholy and Frost Presence with 1 keybind.
/cast Unholy Presence
/cast Frost Presence
Totem Stomp
Rune Strikes (if its up) and causes your ghoul to attack listed totems. Highest priority at the bottom of the list.
/cast Rune Strike
/petattack [target=Windfury Totem]
/petattack [target=Mana]
/petattack [target=Earthbind Totem]
/petattack [target=Disease cleansing totem]
/petattack [target=Grounding Totem]
If you have any additions or corrections, please post them. No more ShadowFrost threads!