Moving to the US, need a sim-only monthly plan (or contract)

Hey people, I'm moving to the United States in a matter of 10 days and would like to get a monthly (rolling) plan or even a 12 month contract if there's no other choice. I already have an iPhone but I may buy the new iPhone, but at full cost from Apple.

Can I have some advice on some non-phone monthly plans from T-Mobile/AT&T or any other carrier of your choice?

PS: I've an unlocked iPhone 4S, purchased from AppleUK.

For someone peeping in without an account, please leave a reply @KartikThapar.

Thanks all!