I took them up on this offer today at my local Gamestop. Sadly they didn't have any Slims in stock so I will have no console for a couple weeks.
I traded in a 360 Arcade, second 360 that was broken, 20 gig hd, 4 wireless controllers, 1 wired controller, Wired headset.
4 360 games:
Sega...
I ordered an Arcade via Dell.com last week and it arrived today. It is a Jasper unit for those interested. Though I'm not sure this deal is still active.
Edit - I got a 150w PSU with mine for what it's worth.