Here's what you do:
1. Get a US iTunes account
2. Plug in your iPhone to activate
3. Enter some US Address
4. When asked for a SS #, just enter all 1's or all 9's and submit
5. Select the helpful PrePaid option at the next screen
Link to TUAW.com story:
That SS will guarantee a rejected credit check at which point iTunes will give you the option of starting a Pay as You Go plan for your iPhone. I did this for a friend based in London and FedEx will deliver his iPhone on Tuesday.
It is actually a "Pick your Plan" option, but still no contract. The cheapest Pick your Plan is $50 a month, I believe, and you can stop it anytime.
If you stop the monthly payment you can still use the iPhone over WiFi and as your iPod.