We've tried a few times using voice over internet and cheap web cams. The results were mixed with the largest problems being a slight lag that definetly kept the electronic players a step behind and a tendency to ignore the e-players, especially when we used a split comp screen to show more than one player.
The problem ultimately was humans simply ignore the ones who aren't there, especially when things get heated. It worked as an occasional thing, but I wouldn't want to run a game like that on a long term basis (though it might get better with experience).