Do any phone carriers offer prepaid micro-SIM cards for unlocked iphone 4 / 4s? Or am I stuck with $50+/month data plans?

I’m just curious what the purpose of buying an unlocked iphone 4 is if I still have to go through the carriers to get the “phone” part to work. I’ve done pretty well with a iphone 3G / 3GS just using a prepaid SIM card for phone, and wifi for data. I’d like to continue, but understand the 4 /4S now use a micro-SIM, just curious if anyone has any experience with getting micro-SIM’s from cell carriers for prepaid use. Alternatively, if there are any reasonable cell/data plans for iphones, I’m all ears.