I just make iMovie for iphone 4 run on an iphone 3GS, it's very simple 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
Take a look at the video below posted by RedmondPie. They've posted some instructions as well; however, the installation of the modified ipa requires circumventing Apple's DRM so we can't post a full tutorial.
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![iMovie Runs on the iPhone 3GS Just Fine [Video] iMovie Runs on the iPhone 3GS Just Fine [Video]](/images/news/10343/418226/418226-640.jpg)