We've got an iPhone with a UK SIM, but we're in the US for a month and need a US sim to avoid stupid bills for wireless internet. We found you can buy special phones with a pre-pay account unlimited for a month, the phone is like $20 so we planned to buy one, throw the phone away, and put the SIM in our iPhone... but it seems you can't do that... the iPhone is unlocked but the mobile provider has the SIM locked to only work on their handsets.

So, since we obviously can't get a contract, can we just buy a SIM and charge it up for $30-50 or something? We couldn't see it in Wal-Mart, would one of the cell providers do this or would they insist on trying to sell us a cell+contract?

You can get prepaid phones for cheap. They charge a small activation fee and then you pay per minute. I just bought my brother in law 400 minutes + activation for $100. I'm not sure how much the phones cost though. My guess is around $50.

They definitely have these at walmart, target, and every other store with cell phones. You must not have looked at the right place.

You didn't read my post. I already said I found cheap phones with plans like that. The problem is I need to be able to put the SIM in my existing iPhone, and they don't seem to allow this... the SIM will only work in pre-selected phones.
The cheap phones are great just for making calls but to use for mobile browsing (like walking around using google maps) I'd end up spending more on a better phone I only need for a few weeks.
So, does anyone sell a pay-as-you-go SIM without a phone?

You didn't read my post. I already said I found cheap phones with plans like that. The problem is I need to be able to put the SIM in my existing iPhone, and they don't seem to allow this... the SIM will only work in pre-selected phones.
The cheap phones are great just for making calls but to use for mobile browsing (like walking around using google maps) I'd end up spending more on a better phone I only need for a few weeks.
So, does anyone sell a pay-as-you-go SIM without a phone?

I'm not aware of anything like that. Even if you could get a sim, you'd have to get a data plan to browse the internet (or pay outrageous browsing rates per MB). It would probably be better to stop at a Starbucks or McDonalds and use wifi on your iPhone when necessary. Most bookstores and coffee shops also have wifi access.

That's a shame. The annoying thing is, those cheap phones you mentioned come with data in some cases... one we looked at you could get unlimited calls, texts and data for 30 days for a one-of fee of $50. But a phone to make the web useful would cost quite a lot and you can't transfer the sim.