There is another fully discussed thread on this very topic, so a search might give you what you need. My understanding is you give ATT $15 & they will give you a SIM. However, you will need a CC w/ a US address. The work around is to buy a VISA gift card and use the address of your hotel.
I called AT&T and they gave me a line about having to contact Apple for a "replacement" micro sim, or as I like to call it a spare for use in whatever I choose. Anyway, I know that other people have in fact gotten replacement micro sims from AT&T, so I'm not sure what they were saying unless they were trying to deter me from getting a spare. ANYWAY, I searched on eBay, found one for $22 with free shipping. Ordered it, got it, put it in and signed up for my spare service. Also, since I pre-ordered my 3G and got it delivered on launch day, I obviously signed up for the unlimited plan for $30. Well, to my utmost surprise when I was signing up for service on the iPad, it remembered my iPad as being on the unlimited plan, so I had 3 options: 250mb, 2gb and unlimited for 29.99!!!! I am a happy man. Didn't know how it'd go, but it worked awesome for me. Maybe TMI, but had to express my joy. Hmmmmm, also sounds like I have unlimited data for my no contract iPhone 4