Some people have apparently been having problems using USB headsets in World of Warcraft voice chat. Here's the proper steps though:

1. Plug in headset before loading WoW.
2. Load WoW.
3. Select USB Headset for voice in/out and/or audio out (choose what you want for your outputs).
4. Reboot WoW, and it will work (It even says you ahve to reload WoW to have these changes occur)