I need a US SIM Card for UK Mobile...Help?

Yeah, it needs the micro-sim. I know here they wouldn't do PAYG sims for the iphone 4, so it may be the same in the US. Not a huge deal, though.

I know my stepdad wants to/has upgraded his iPhone 3 to a 4, so I might be able to steal the 3 to use, if he hasn't gotten rid of it.

http://www.t-mobile.co.uk/shop/iphone/micro-sim/

So you might be lucky in the US, otherwise...hacksaw time