If you're looking for a video editor for iPad that offers a little more flexibility than iMovie and runs on a first generation device, Avid Studio has more options and control than other iPad offerings.
The bulk of the app is remarkably similar to Apple's iMovie app for iPad 2, but Avid Studio has a few notable features that give you more control over your video. This includes a variety of montage templates, precise edits, layering and a simple, but well implemented audio editing tool. The drag-and-drop interface for throwing movies in a timeline works as you'd expect and the frame-by-frame editing feels more accurate than iMovie. It's not going to replace a desktop application, but it will do the job for short and quick edits.
Avid Studio works on the first-generation iPad, but users are complaining in the review section in iTunes that the app crashes when attempting to render video. I didn't have trouble myself, but I wasn't trying to render anything more complicated than two short clips mashed together. Avid Studio is $5.49 in the iTunes App Store.
Avid Studio [iTunes App Store]