Walmart is offering store credit on older handhelds, redeemable against the 3DS.
http://www.nintendoworldreport.com/deal/25776
For the first time, Walmart will offer store credit for all versions of Nintendo DS consoles, redeemable against a purchase of a 3DS at launch. Older consoles will be accepted at various trade-in values for credit towards a 3DS at participating stores.
The "Take-Back and Trade-Up" is set to run as a trial from March 27 to April 30 in 12 states. The store credit values for each console are as follows:
DS - $20
DS Lite - $50
DSi - $75
DSi XL - $100
This is the first time that Walmart will have a hardware trade-in program. Full details of will be revealed later this week, and the values are subject to change prior to the official announcement.
UPDATE: The states included in the program are Alabama, Arkansas, Florida, Georgia, Kentucky, Missouri, Mississippi, Oklahoma, North Carolina, Tennessee, Texas, and South Carolina.