I think Wi-Fi is the way to go. 3G is slow, and you're always going to be counting the MBs to make sure you aren't charged an arm and a leg by going over. With an internet only (nearly) device, 100 MB/month probably wouldn't come close to lasting 30 days for most people.
Remember, the cloud is supposed to change the way we think about computing. 100 MB will probably be the "emergency only" choice, not the bread and butter.