Despite iOS 4 coming to older model iPhones, iMovie is still only available on the iPhone 4. It turns out, though, that you can get it running on a jailbroken 3GS with just a minor plist edit.
We've shown you how to get multitasking and home screen wallpaper on your iPhone 3G, and while iMovie won't run on devices that old, you can get it up and running on an iPhone 3GS — that is, if you still jailbreak your iDevice (and if you've upgraded to iOS 4).
Over at tech blog Redmond Pie, they've discovered all it takes to get iMovie working on a 3GS is a few tweaks in the app's plist file. They don't go into detail on how to edit the plist file, but we've covered SSHing into your jailbroken device on more than one occasion — once you get in, it's just a matter of using OS X's built-in plist editor or a third-party editor for Windows to tweak the minimum system version line and the front-facing-camera line to "3.0.0" and "false", respectively. Hit the link for more details, and let us know how it works for you in the comments.
iMovie Ported to iPhone 3GS, iOS 4 [Redmond Pie]