Over at Kara-Moon they have been experimenting with "ninjam" for jamming back and forth over the web.
It keeps track of the 'loop' being used so people hear the audio sync'd, although a few seconds later.

As far as jamming on skype, if you thought MME latency was bad ... well it would be a struggle, even though it seems in 'realtime'. However the HD TV concert idea is pretty inviting. That would be cool to broadcast a performance over skype.

I do not work here, but the benefits are still awesome
Make your sound your own!