I got a Nintendo DS for Christmas and I must say, as a casual gamer, I love it! The touch screen is a nice touch, and a Wi-Fi web browser is on its way.
I got MarioKart DS, probably my favorite game, and not only is the game play fun almost never boring, but the Wi-Fi play adds whole new element. Let me know if you want to play.
Advance Wars: Dual Strike is unbelievably addictive and fun. It's a turn-based strategy game that I can play in short stints or for hours at a time.
Final Fantasy III is as fun as it is punishing (save points are non-existent). I've never been much of an RPG fan, but I was a big fan of the Final Fantasy III (which I think was actually FF IV because of Japan-only releases) for the SNES and this is a nice little throwback.
Next I want to give the new Super Mario Bros. a try and perhaps Elite Beat Agents... anyone else have a DS?
If you're a battery, you're either working or you're dead....