That day is today.
I don't know why they did this, but in iMovie HD, all vertically-oriented photos get rotated back to horizontal orientation, and there is NO way provided in the software to rotate them back.
Sure, I could import them into a photo editor, lay them over a horizontal black background, save them out, then pull them back into iMovie....but come on. I just wanna make a stinkin' slideshow with some videos intercut here and there. What happened to the ease of use that they keep touting in all their ads?
I could've sworn that I used some vertical shots in a movie project a while ago, but for the life of me I can't remember how/if I pulled it off, and an hour (or more) of Googling has not yet led me to a solution.
That said, the evening wasn't a total waste. I did manage to develop a headache! Thanks, Apple!