Trade-in offers are rather common -- many retailers, including stores other than Gamestop, will take in old equipment in exchange for money or credit. This includes Microsoft -- which will currently take your old 360s...and your old PS3s, too.
In fact, there are reports of Microsoft emailing this ad to people:
...which almost makes it sound as if it's a PS3-only offer, when it's not. According to the Microsoft Store website, you can trade in the PS3, Xbox 360 S or Xbox 360 E for store credit. The consoles have to work for Microsoft to take them in, of course, and the offer is only good until March 2nd in the United States.
Its cool that they're not discriminating and are letting people trade in more than one last-gen console, but...it's also kind of weird, no? Just what are the Microsoft Stores gonna do with all the PS3s they get? Its not like the Microsoft Store sells PS3s. I can just imagine sad stack of boxes of PS3s in the back of Microsoft stores...aww.