Finally figured it out: I should have paid more attention to the suggestion of looking at the dust shield, because that's exactly what it was. After getting a second set of calipers (this time from Napa) and changing them, I thought about the sound I was hearing. It was constant, even when I was braking. In my mind, if it was the pads making the noise, the pitch/tone/sound in general should have changed. So this time, when I put the new calipers on, I took the rotor off first. And lo and behold, there were some nice scrape marks on the shield as well as some spiffy grooves in the backside of the rotor. I banged down the shield with a hammer as much as I could, and the scraping has all but disappeared. I get it every now and then when I'm turning, but no biggie.
My guess? Well, as I mentioned before, the previous rotor was a composite rotor and the hub measured out for a composite rotor, but the bolts always seemed longer on mine than what was in that picture that was in the brake rotor thread linked to earlier. And that always bothered me. I'm thinking that the hubs got replaced, maybe the bolts were swapped out... needless to say, I'm going to take it to a mechanic to get the drums replaced, because I'm tired of dealing with brakes for a while!
Speaking of mechanics, any suggestions on a good place to go and have the drums replaced? I could go to the dealer, but figure they're going to be more expensive than some other place that can do just as well.