Mmm, the integration of MSN and Yahoo may not be exactly what you're envisioning, as far as I can tell. I finally got around to downloading this and trying it out.... I'm not sure yet if video is possible at all, but in terms of the compatibility itself, it's not quite like you put your MSN credentials into Yahoo Messenger and it logs you into both services. Rather, you have to add Yahoo contacts to MSN or add MSN contacts to Yahoo -- individually. So the practicality of this solution seems somewhat limited.
Nonetheless, I'd like to try videoconf with it to see if it's possible at some point. The docs are not clear about this.