I just wanted to add an adendum to the N-Gage thoughts below. Many games that have been launched already have support for the N-Gage Arena, which means you can connect online within the game and participate online with other N-Gagers. It's actually really damn cool and it *works* perfectly. How many online mobile services can you say that about?
Right now the capabilities are pretty limited, but in Tomb Raider for example, you connect via GPRS (free for the next month using the included T-Mobile card that came with the phone) and are able to see a menu of different clips you can download. You can download clips that others have recorded as well as tips for hard areas such as where some of the secrets are. The downloads are really fast - I assume they're just instructions to the game, and not video clips - and you can instantly view the clip of the game with VCR-like controls. It works really well. Also, you can "shadow race" against other players by downloading a clip which is played back at the same time you play the game, with the recording displayed as a shadow that you have to beat.
It's not a full-on online multiplayer, but the fact is that it was pretty gee-whiz to me and I'm getting harder to impress. So the device sucks (though Jock spent the past hour telling me that it all has to do with the games and just wait until John Romero's Red Faction hits) but the Arena service is pretty damn neat, and supposedly is just starting with more functionality coming soon.