Check out iVisit (http://www.ivisit.com). It's a little program that offers cross-platform video. I remember that the video was quite good (though you have to pay for the app, I seem to remember it wasn't a huge amount). Anyway, you'd both have to buy it, but worth a look.
Mercury Messenger actually supports receiving video , sending not yet , but danny(creator) has found a library that could capture iSight , so sending would work in a few months