Cannot export with iMovie on iPad. Memory issues
Not sure if you've found the solution or not, but discovered from this link: http://rockonflash.wordpress.com/2011/04/28/imovie-eating-memory-on-ipad/ indicated that if you make a change to the original project it frees up your space.

Additionally, I've read that it's better to save/export the project to iTunes instead of locally, and then distribute to where you need it.

Hope this helps you, or any other poor traveler who runs into this issue.