Oh, and since it hasn't been linked here:

http://apple.com/mac/facetime - Location of the beta for OS X. I did a limited test with it this weekend, Mac to Mac, and it seemed to work fine. It is a little weird that Facetime will ring even when the app is not open, as Apple added a push message receiver to OS X. I'll have to do some testing to see if an incoming Facetime call will wake a sleeping Mac on a Wake on Demand network.