I regularly use audio and video on Yahoo Messenger with general success.
Admittedly I don't use a combined video/audio cam - seperate Webcam and Mic into a soundcard.
One thing I have found is that you absolutely have to implicitly allow the Yahoo voice servers through any firewall.
I have a list of their current voice servers if you want to give it a go - there are about a dozen.
Cheers
LL34