How do I delete just the videos from my iPhone?
Sorry for the self-answer, but I found a way to do it quickly using just the iPhone and no third-party software.

In iOS 4.3, do the following:


Open Camera.app
Click the photo thumbnail to view the photo library
Click Camera Roll to view all photos
There's a capsule button at the top that says 'All|Photos|Videos'. Select videos
Press the Export icon - you can now tap on videos to select them before hitting Delete.


Not sure why this functionality is not available from Photos.app, but there you go.  Thanks for all the other answers.