Least expensive prepaid data roaming with an unlocked iPhone in the US

Solution 1:

My solution was to activate an iPad MicroSIM and use it in the iPhone. It required a bit of trickery to get it going but worked like a charm - $25USD for 2GB for 30 days

Activating an iPad MicroSIM from Safari (when you don't have an iPad to use): http://modmyi.com/forums/unlocking-activation/712894-activating-ipad-3g-sim-without-ipad.html

Solution 2:

T-Mobile no longer provides (officially or unofficially) prepaid $1/day data access for anything except the actual Sidekick device, nor do they have any other prepaid data options. They do offer no-contract postpaid voice, data, and texts through their Even More Plus plans, but you need to set up an account, billing, etc for these plans.

If all you're looking to do is get online, another option is to rent a MiFi. It's a little pricey, but you can use it with your laptop, iPhone, and whatever else you have that has WiFi.

If that doesn't work for you, then it seems that you'll have to either go with AT&T, use Rogers' data roaming, or try to find a prepaid data plan through a regional carrier.