GameStop members can now trade in their DS Lites, DSi, or DSi XLs for a reduced price for the upcoming 3DS.
Not sure the exact prices at the moment, but I'm pretty sure DS Lites come to $199.99 for a 3DS, and then DSi's come to $150.99 and then DSi XLs come at $129.99.
It also depends on how your DS functions (if it's new or is somewhat damaged, etc.)
I heard something like this before. I don't think my DS is worth anything because of it's broken top screen, but I plan on keeping it for GBA games, anyways.