Along with the release of the app, Apple has posted a list of frequently asked questions, as well as instructions for exporting videos from iMovie. Curiously, while the instructions detail how to export your videos in full HD (at a resolution of 720p), the summary at the top of the page suggests that your video will be scaled down to 568x320 if uploaded to YouTube, MobileMe, or sent via e-mail from the phone. A third support document posted this evening further confirms this to be the case.
Since I'm not one of the lucky customers who already have their iPhone 4 in hand, I can't directly test this, but it seems that Apple has spelled it out pretty clearly in their support pages. While this won't be a deal breaker for everyone, I'm sure that many HD aficionados out there will be longing for the ability to directly upload HD video to YouTube from their phones. As it stands now, you would have to sync the video to your computer, and then upload it from there for maximum quality. It is worth noting, however, that this limitation only applies to using the sharing functionality in the Camera Roll on your iPhone; any third-party apps that can upload video should be able to take advantage of the full resolution copy.
So aside from the resizing, iMovie for iPhone 4 looks promising. And at US$4.99, it is an inexpensive way to extend the video capabilities of your phone, allowing you to quickly create and share movies of friends, family, or special events while on the go. If you're picking up a new iPhone tomorrow, this is definitely something to check out.