An excellent post from iPad Insight on how to get your videos from your iPhone to the iPad without having to sync your devices. Quite a few options here :
How To Get Videos from iPhone to iPad Without iTunes Sync.
It seems too that for iPad users of the iMovie App you are limited to editing movies created either on the iPad or iPhone. Hopefully that changes with future upgrades.
See the support sections of Apple:
iMovie for iPhone and iPod touch support page
Can I import an iMovie project on my Mac into iMovie for iPhone 4 and iPad touch (4th generation)?
No, the projects from iMovie on your Mac cannot be imported into iMovie for iPhone4 and iPod touch (4th generation)
(eds note - this seems to apply to the iPad too)
iMovie for iPad help