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One of the coolest new apps in the iTunes App Store is iMovie. You can use this $5 app to edit movies you shoot with your iPhone camera, add music, transitions, and other effects, and export complete videos which you can upload to YouTube and other sites. The only catch is that it’s only available for the iPhone 4… officially.
Redmond Pie has figured out how to get the iMovie app to work on a jailbroken iPhone 3GS running iOS 4 (http://www.redmondpie.com/imovie-for-iphone-4-on-iphone-3gs-video/).
Technically the iPhone 4 has a faster CPU than the iPhone 3GS, which is one of the reasons iMovie isn’t available for download on the 3GS. But as you can see in the video below, while iMovie on the 3GS may not be up to Apple’s standards, it does appear to be reasonably fast and responsive. While the same trick should work on the iPhone 3G and iPod touch, iMovie will likely run slower on older devices.
You can find instructions for altering the app to run on earlier iPhone models (and maybe even earlier iPhone OS models) at Redmond Pie (http://www.redmondpie.com/imovie-for-iphone-4-on-iphone-3gs-video/).
iMovie on iPhone 3GS and iPod touch on iOS 4 (http://www.youtube.com/watch?v=N6L07nPrjzo&feature=player_embedded#normal)