I've heard only rumors of the "online" capabilities of the DS. I've heard that, as it is now, it's compadible with a Wireless G Router -- but the games aren't. It's going to be an amazing game feature, though. It's one of the few "online" systems I'll like because of one thing -- I'm already paying for DSL (thankfully Sprint finally got it out here, dummies), and the games won't be "pay to play" (which I hate about most online games, now).
I know several games that, given online capabilities, will be plain awesome. A DS SIMS Online could be quite fun, and I'm curious about how a Pokemon would be pulled off.