iMovie allows you to edit any video recorded on your iPhone 4 device. This software was released by Apple and restricted to only the iPhone 4, which makes sense as it’s insanely faster and can record in 720p HD.
However some folks wanted to use it on their iPhone 3GS, but since there were restrictions on the installation of the package it wasn’t possible. You can get around this restriction by modifying the plist.info within the Application. Exact instructions on how to do this can’t be provided as Apple will take action. Here’s the instructions.
“just download the file, modify info.plist file and change the Minimum system version to 3.0.0 and UIRequiredDeviceCapabilities/front-facing-camera to false”
Redmondpie has put up a video of the iMovie on a 3GS.

It Works. thks.