If your thinking on getting the Flip HD for breath taking HD quality, you can forget that. I have a Flip Mino HD, it's far from what you probably think the video quality should be...but then again I know the limitations of the Flip HD and I'm not complaining. I use it for what it is, fast, small, and easy to use.
From what I have read the iPhone 3gs can record 640x360 (VGA, SD) video. Just because it records at that pixel dimension doesn't mean the video quality can't be better than HD video. The Flip HD records at 1280x720, and that's why it called HD.
If the video quality of the new iPhone is better than the Flip HD...go with the iPhone.