iLounge has recorded 6 videos comparing the video recording capabilities of the iPhone 4 to those of the old iPhone 3GS and Flip Ultra HD. 3GS aside, the Flip Ultra HD has almost the same quality of the iPhone 4 (they both output 720p video) and better saturation but, like Gruber points out, the iPhone 4 wins in moving and panning.
Check out the videos embedded after the break.
iPhone 4 Vs. Flip
iPhone 4 Vs. Flip, Video 2
iPhone 4 Vs. iPhone 3GS