It was not too long after it was released that a developer figured out how to do it.
How to Install iMovie on iPad 1
2: Run iPhone Configuration Utility, click on “Applications” and then open iMovie (found under iTunes / Mobile Applications)
3. Connect first-gen iPad via USB cable and select your iPad listed under “Devices”.
4. Now select the “Applications” tab and hit the “Install” button and iMovie 1.2 would be installed on to your iPad.