This is a rumor at the moment, but perhaps it's grounded in fact.
Apparently someone spotted the Sims for GBA on EA's release list at some point, and it's not there anymore. Still, this could easily be a game in the works. Why? Well, despite the hurdles involved in making a game like that work on such a small system, the Sims is a goldmine of gaming dollars on the PC. It only makes sense that EA would try to port it in some way to the GBA. For now, this is nothing more than rumor though.