Going back to the basis of this thread, the original idea was how to get telephony sound without using a Gentner or other expensive interface. While the interfaces do the job very well, some are confusing to set up and others simply too wiz bang for what most are trying to do. If your going to send video with audio Citrix, Skype and a few others are the best way to go. Shoot, if your going to go that route you could even simply run multiple live instances of Ustream.
A way of doing audio only, would be a VoIP converter as I mentioned earlier. For cancellations I think it depends on user setup. You could invert the polarity of one side of the audio conversation and that might do most of the work for you. I think that most converters are going to work the cancellations out for you already though. Not to mention that if you designed and deployed the PA well, there will be minimal issues from loop back anyway.