It turns out it wasn't hard, but it wasn't that easy, either.
Perhaps my biggest complaint is that when you are assembling your edit, iMovie provides a scrolling list of "strips" representing the clips you have. But it doesn't indicate which ones you've already used, or let you flag ones you like or dislike. This means that you have to keep track of all that yourself; a problem if you have dozens of clips that look similar.
iMovie media browser
Once you have completed your project, you Export it. You have the choice of three sizes: Medium (360p) Large (540p) and HD (720p). Exporting a 1:30 long clip at 720p takes about two minutes, and places the movie into the Photos app Camera Roll. Going to the Photos app, choosing the exported clip, and then choosing Export from there gives you the choice to email or upload the clip to YouTube. It then recompresses the clip a second time - at least if it's in 720p - before uploading. That recompression is much faster, maybe taking 10 to 15 seconds.
I had no WiFi at the SuperMeet, and had little success uploading via AT&T; the upload failed four times. I'm not sure why. The progress bar would chug along for a while, and then it would stop with a dialog error that the upload was incomplete. I then went in search of a public WiFi spot, but had little luck. Oddly enough, about an hour later in the auditorium for the evening meeting I tried once more, and the upload worked! I have no real idea why. My only guess is that the signal strength might have been better in that location as the speed of the progress bar seemed faster...
The final movie uploaded from the iPhone is in 360p and heavily compressed. That's the least of it's issues; the color of the clips is pretty bad in a lot of places! Of course, the video was shot indoors in less than ideal conditions - though typical for casual indoor shooting.
I have also uploaded for comparison using my computer the 720p version that the iMovie app placed in the Camera Roll.
SuperMeet video in 720p
SuperMeet video in 360p uploaded from iPhone 4