The guys over at Redmond Pie have managed to modify the iMovie application file and install it on an iPhone 3GS. As you can see from the video after the break, it works just fine, despite Apple selling it as iPhone 4 exclusive for its high RAM requirements.
I assume that to do this you’ll have to edit the .ipa file of the app and enter a few strings to add 3GS support, then move the app into your device via SSH or iTunes. Redmond Pie removed the step-by-step guide due to Apple DRM violation, so I can’t tell. If you installed iMovie on your 3GS too, please let us know.