iMovie won’t install on your iPad1? Ok, now I’m confused. iPad1 has no camera at all, I know the original iMovie wouldn’t install. I’m running iOS 5.1 on a never-jailbroken iPad1 64 GB Wi-Fi only.
Downloaded iMovie 1.3 on it last week, installed without any protests.
I know iPhoto won’t work on it, yet.
Same configuration, different results. Maybe you caught a short window where the restrictions were looser while they were adjusted to include iPad gen3. Alas...