Some video cards cannot support Ventrilo's built in overlay feature. This may be what banjerkid is referring to.
Here's an additional program that you can run with Ventrilo, so that you can see who's talking while still playing in a regular, fullscreen mode of WoW.
http://www.voice-overlay.info.ms/
I have no idea what happened to the project author, though I guess the program works fine.
I've used it briefly without having any problems.