If you're running a 64-bit OS with the appropriate hardware to match and consider yourself a World of Warcraft fan, a new 64-bit client is available to test.
Whether you're the testing type or not, with the 64-bit version of the game you can look forward to potentially better loading times, increased stability, and more.
Hit the source to grab it, though be warned it's for the test version only, meaning you won't be able to play live with it. Voice chat won't work, also. Some users have had trouble getting it going; if you run into issues, check the PTR bug forum for advice.