I wouldn't see any issue with this. Say someone does buy the DS and it doesn't load your regions games. Big deal, they switch it out on the spot for one that can, and the manager sends it into GameStop so they can ship it to a country that can play said games. Just because you trade it into your local region doesn't mean a world-wide company can't deal with this problem. And this promotion is happening world-wide. I would cash it in and if you're really worried about it, tell the manager they need to ship it to a US Gamestop for inventory as it's NTSC region.
Edit: Not to mention, a lot of promotions like this end up sending them into IT recycling, while keeping the pristine units that were traded in for resale.