hi, u can buy 3g iphone in US, but Apple said u have to sign the contract with AT&T together, whatever u buy from Apple store or AT&T. Also all cellphones are locked in USA, cos they want to protect US manufacturers. I think u can buy from ebay, most of them are unlocked, u can use it in HK.
the size of SIM card is the same size as HK, no need to cut it.