Getting a SIM short-term in the states is close to impossible. Any provider that uses GSM (Cingular and T-Mobile are the only US Nationwide carriers that fully use GSM) is going to require a credit check and an at-least 1 year contract. Additionally, like everything else (electric, NTSC, etc), we don't use the same GSM frequencies that the rest of the world uses, so your current phones would need to be quad band GSM to actually work on our networks.
If you have a quad band handset, your carrier over there may actually allow you to roam onto a US network here. I'm sure it's not cheap, but they may very well allow it.
Simply put, I'd invest in some radio's