I just signed up with TypeFrag.com for a Ventrilo server that our guild Boulderfist Heroes can use. I run WoW on a Mac and for some reason every time I would try and talk over Ventrilo, it wouldn’t work unless Ventrilo was the active application. This allowed me to listen but never say anything. Well after searching Google I finally found the answer (thanks Kalei on Twisting Nether)!
Make sure under System Preferences, Universal Access; you have the option to “Enable access for assistive devices.” checked. That allows one app to control another one. Vent on a Mac is finicky though but I’ve got it working fine. After you make this change you will need to quit the Ventrilo client and launch it again.