Is it possible to screencast just one desktop in Spaces?

I'm considering using something like Camtwist to capture my desktop and stream it live to Ustream.tv. I was wondering if there's any way to record just the one desktop so I could switch to others without them being broadcast. (Say I had iChat open in one space and Photoshop open in the one I wanted to capture. By default it shows me switching between the two when I go to answer an IM. I'd like it to stay on the PS space so it wouldn't be interrupted.) I wouldn't be surprised if there isn't a way to do this but I thought I'd ask