I'm not convinced that people would be interested in a data plan for a gaming system ...
Most people see smart phones as universal communication devices because you can talk, text, email, use chat programs, update facebook, twitter, etc; and most of the functionality of these devices requires a data plan. Now, I could be wrong but I suspect that if someone wanted to pay for a subsidized electronics device for communication purposes they would probably choose a smart phone of some kind.
This means that for the data plan to be meaningful for the NGP people would have to want it for a game related function; and this would be for playing games online or downloading game content. From what I know about 3G (which isn't much) I don't think it is well suited to playing games online, it is a relatively expensive service for someone to pay just to play games online, it would be painful to download games due to the bandwith, and in many countries a single game could push an individual passed their monthly bandwith limit and end up costing them a lot of money.
