So, pull out all the plants, etc., to get to the gravel. Sure enough, no shrimp. All four of them, simply gone. According to the chick at the pet store, the fish wouldn't attack them, but if they died the fish would probably eat them. I assume this is what happened, but I dunno why they would have died. They had plenty of food down there...maybe they just overfed or something.
So, my "little shrimpy boys" are no more. To keep the bottom clean of extra food, we added three little albino catfish, and, I gotta say, they are pretty nifty looking against the black gravel. I've been watching them nose around while doing some major file-burning today (6 DVDs full of video files and comics, and more to go!).