1) Open up your WTF folder in your WoW folder.
2) Open up Launcher.WTF in notepad.
4) Open up Config.wtf in Notepad.
6) Start downloading the 300mb update by starting Launcher.exe
Found this on some other forum, hope this might help (afaik this isnt against the TOS, and it might give you a small head start on your journey to 85 when cata hits)
+rep if it helped :)