Webcam/Video Multiplexor Software

I want to stream live video on ustream and mix-in (multiplex) various webcams (And preferably screencasting from one of my monitors).

ustream provides $200 software for this task, but the only feature I'm really looking for is a means to automatically cycle between various sources.

I imagine someone may have a small program that just aggregates together multiple sources and provides a software-dummy video device that gives the output from whatever source is being committed to output. I've searched around for this and not found it, but I may not be using the exact right language.