Funny you should mention this... I was working on making our trailer for our upcoming game today. As far as screencasting goes, I bought
IShowU HD today and made this video with it (and iMovie of course):
YouTube - RoboArena Trailer
It was only like $30 and had a bunch of nifty features. I can recommend it. I was using Jing, but it takes TERRIBLE quality video at a terrible frame rate, so that was a waste of money on my part.