A $20 "unlimited" data plan that can only be used by the 2G iPhone and cannot be used by a laptop doesn't sound like such a great deal to be honest - you're hardly likely to rack up much bandwidth with such a pitiful cellular connection, so the carriers aren't giving much away. The $20 deal looks good on the surface but when you realise the shortcomings of the iPhone to which this plan is locked it loses some of it's sheen.

On the other hand in the UK, for $25 (on top of any voice plan) I can surf a countrywide 3G HSDPA (3.6Mb/s) cellular network (T-Mobile) using my laptop or NIT (or desktop PC and bluetooth dongle!) with a 3GB monthly cap. Now that's a good deal, and is good enough for some people to give up their landline based broadband connection entirely.

A US carrier offering 3G/HSDPA unlimited - realistically speaking this means a fair use cap - access for ~$20-30 would really be something, particularly if "laptop" access is allowed. I doubt it will happen any time soon though as the US cellular networks seem to be a long, long way behind the European networks for some reason.