Apple doesn't want you to use iMovie on the original iPad because they don't think there's enough processing power and memory for it to work well enough. But whatever! You don't have to listen to them! There's a way to install iMovie on your ol' betsy iPad without even jailbreaking.
Here's how, courtesy of fscklog:
• Install the latest version of iPhone Configuration Utility 3.3 (Mac, Windows)
• Download iMovie from iTunes
• Open iPhone Configuration Utility
• Click applications on the left side and click open iMovie
• Connect your original iPad to your computer and click the install button