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