Want to know how to install iMovie 1.2 on to the first generation iPad and that too without jailbreaking it? Follow these instructions (very carefully):
First, you will to download iPhone configuration utility. Start the utility you just downloaded; click on "Applications" and open iMovie – you can find it under iTunes > Mobile Applications.
Now, connect your iPad (first generation) through USB cable and select your iPad (which is listed under "Devices"). Select "Applications" tab and hit "install" button to install the new iMovie v1.2 on v1 iPad. That is it!