According to a report by Jeff Carlson of Tidbits, "sources within Apple" have reportedly noted that the new $4.99 mobile iMovie for iPhone requires the faster A4 processor in iPhone 4, but will not work on the A4-equipped iPad, at least not at the app's launch next to iPhone 4 on June 24.
Carlson wrote, "I suspect the app is tailored to the iPhone 4's higher-density screen, and therefore wouldn't work within the iPad's pixel-doubled compatibility mode," but Apple is likely to support high resolution iPhone 4 apps as running at their native resolution on iPad, where they consume exactly the same real estate on the screen as a pixel doubled iOS app designed for earlier iPhone models.
More likely, iMovie for iPhone won't work on iPad right out of the gate because it takes advantage of operating system features within iOS 4, which won't be supported on iPad until later this fall.
Carlson also reported that iMovie projects won't transfer to the desktop version of iMovie for additional editing, although the movies it renders are in standard formats that could be imported into a desktop project. Movies can also be directly exported to YouTube, MobileMe galleries, emailed or sent as an MMS message, in Medium 640x360, Large 960x540 or HD 1280x720 formats.
The mobile app uses videos from the standard iPhone 4 camera roll, so it appears that externally shot video could be used as long as it conforms to the same standard formats for video shot by the iPhone 4 front and rear cameras.