Very cool idea. I would definitely use something like this. I only noticed 1 small issue: it seems tabbing between input fields doesn't register. As soon as I entered text on the first input, I tabbed to the second textarea and entered text. The video didn't register the second textarea text until I had clicked one of the radio buttons (using Safari 3.1.2 on Leopard 10.5.5)

Perhaps you could also work in goals somehow. This is one of the best things about usability tests--giving users a set of goals to complete and watching how they respond. Maybe offering certain users a chance to take place in the study with X reward?

Also, CrazyEgg (another site for visualizing clicks but implemented as a heatmap) lets you run campaigns, which is extremely useful. Considering you generally want to run tests like this after you've made changes, having a set limit is a good idea (10 hours, 100 users, etc...).

One small critique, I got bored with the video and went straight for the demo. I'd suggest making the video shorter or somehow showing "the goods" up-front.

Very nice idea and good implementation.