The point of unlocking an iPhone is to be able to use it in other countries with GSM-compatible carriers.
For example, when I moved from the US to Canada I had to unlock my original iPhone for it to work up here. I do in fact use prepaid cards with it, but that's because the Canadian carrier I chose is compatible with the iPhone.
I don't know if Boost Mobile would work, but I seem to recall that in the US only AT&T and T-Mobile can work with unlocked GSM iPhones (the Verizon iPhones are a different thing and only work with Verizon at present).
For the record, I unlocked my iPhone after my AT&T contract was up and after that they were perfectly happy to put me on their "GoPhone" plan (prepaid).
So **IF** you can unlock the iPhone (not all versions are unlockable; depends on what version of iOS and baseband version they are running), and **IF** you're not on contract, then you CAN use an iPhone to switch carriers and use SOME prepaid companies.
