Google will push full VOIP usage on these, meaning no voice/sms plans needed at any carrier. Voice calls will go through Google Voice on Data SIM cards and will provide unlimited free voice calling. And SMS is replaced by unlimited free Gtalk.
Finally, Google will provide easy roaming data services in other countries. For example, if you go to Europe, you can roam on HSDPA data networks for the same price of 20€ per month, and if you only stay in Europe for a few days you won’t need to pay for the whole month. You may not even need to change the SIM card.