you can do this now if you want, but there's a certain amount of latency. You can use a stream using places like this...
http://www.livestream.com/ or
http://www.viastreaming.com/shoutcast. You can rent a stream for 40 listeners for $5 a month (more or less). Then, you can use WinAmp to stream up your audio and your listener base can tune in and hear you live. Now, there is latency in this. If you complain about latency from your interface to your computer and back, the latency to a streaming server and out to your listeners is much worse. You can then have your music brought down by another musician who adds to it live. He can hear you, but you can't hear him, because the latency would be too bad. Then, he can stream it up again. In this way, you can play with another musician on the web. But realistically, this method doesn't let you play where you can both hear each other. I'm just posting ideas.