iMovie - iPhone 4
You could edit video before but with Apple's new iMovie for iPhone there are 1,500 new features, so editing movies on the iPhone is probably easier than what most people use on their home computer!
It can edit on 720p HD and add effects and music from iTunes to your vid. You can even export to 720p.
It doesn't come with the iPhone 4 but you can download it for $4.99 from the app store.
>> See the full review here <<