I think this is a great idea and will take off. We're launching a game in the next couple of days (with MOAI). Once its available we'll add it in for sure.
I feel minimal performance hit is an understatement. Assuming that screen captures are taken at a much lower frame rate, there's still disk write operations involved and writing 15x per sec to disk is definitely not minimal.
We use the same technology that Apple uses in its native camera app to write video. That experience is pretty seamless and has been optimized by Apple to not impact performance. We can record gameplay at 30 FPS while leaving the actual framerate at 60 FPS, as all of the games we are integrated with show: http://www.kamcord.com/games
The native camera is the active application when someone is using it, but for Kamcord's recording, the game is trying to render at 60FPS and Kamcord running in the background, how is it the same?
I'm just curious to know more about the "technology" that you are talking about since I'm pretty familiar with iOS's SDK.