When the Xbox 360 S was announced, Gamestop setup a trade in offer that was originally supposed to be good until the 18th of July. Gamestop has taken down the offer. Checking their website confirms that it has been removed.
For those who were hoping to trade up their existing 360 for the new 360 S may be severely disappointed. Although, I'm sure some store somewhere may not have been notified of this change so if you have any luck doing the 360 trade in deal, post up what store is still upholding the deal in case any GB users live near that location.
Found the Google cached page that shows the offer was good until July 18th. This page was cached on Jun 27, 2010. LINK
@JJWeatherman: Found an article about the ad confirming the deal's removal.
@sixghost: Gives a quick summary of why the deal was removed:
" No, the problem was that the deal was requiring you apply the credit you got from the trade-ins toward a 360 slim, but they are so backordered on the slims that people wouldn't be able to get their Slims for 2-3 months. "
That's a really long pre-order list. I hope none of you guys are stuck on that list.