I had not explored the options in iMovie other than the recent movie trailer additions and a little bit of picture in picture. I realized that it had a green screen processor, which means it will composite in one video with another treating the green as a replacement.
So I thought I would try something. This is just a render test rather than a finished article, but it shows the principle.
I built a green studio in Second Life, just some prims, full bright and green. Placed my epredator avatar in the scene, pointed the camera at me and recorded the result with SnapzPro.

Next I dropped the result in iMovie onto some of the Predlets movie trailer footage. I had to adjust the green screen crop but it actually worked!
I bought a new camera for the business today, a HD Kodak ZX3 waterproof minicam (like a Flip HD but more robust). The iphone is handy for video but thought I needed something, still homebrew, but dedicated. I popped a 16Gb SD card in to give a good few hours of filming.
I will be giving it a test at the Gadget Show Live press day on Tuesday. As if there was not enough to see there already !