I can finally add stuff to this thread! Sorry, long time lurker and cheap ass gamer, first time poster guy.
I'm in North Florida (the city with an NFL team named after a kind of cat). I found the Dollar Tree sale thing just like you guys did, but, I think mine had a UMD that's been left off the tally of UMDs that's been kept so far. Mine had a copy of Mr. Deeds, the Adam Sandler movie. Also, was Year One mentioned?
While these could be leftover stock from Gamestop, one thing I think is more likely is that these were all from a Sony warehouse. All of the UMDs in question are ones Sony released. It's possible that Gamestop had these, or at least some of them, then sent them back to Sony for some reason.
I've only been able to check two stores and of those, only one had the UMDs. I only managed to get ten. There's so many more I would like to find. And another thing I've noticed, is that these UMDs have pretty much all found their way to eBay where they're selling for quite a bit.