I know that ustream.tv can be used to stream live from your computer/usb camera/video camera with usb interface, but, the question I have is, if you are producing a podcast or show with multiple participants through skype or multiple ustream accounts, how can you switch the feeds back and forth on one common feed in a cheap and/or open source way?

Any ideas?

I simply ask because I see Leo Laporte do this with his twit.tv site. However, he has a tricaster studio edition and a bunch of other equipment that I will never have...