I'm from Canada and I just used an iPad in the US for 4 days on AT&T 3G. It's far eaiser to stick with Wifi, but here's my story, in case you want to try:
I live in Canada, but I drove to the US and bought an iPad 3G in april. (being purchased in the US it came with an AT&T microSIM).
I went home and activated it on my Mac (needed OS X Leopard and iTunes 9 and an iTunes account, I believe)
Synced my music, photos, etc and used it on wifi only in Canada.
Then I travelled to the US but didn't have wifi where I was staying. I bought a VISA gift card from the drug store for $25. I specifically chose one that had a 3-digit PIN on the back to make online purchases. I also chose one that didn't require a social security number. Then I phoned the credit card company and told them I wanted to register my address to the card to make an online purchase. I just gave them the address of where I was staying.
After that I waited an hour to make sure the VISA card was activated. I then turned on 3G and went to the account settings area and entered all the requested info, including the VISA gift card and US address.
It worked!
Before I left the US I just cancelled the 3G data with AT&T. It was all done on the iPad itself.
Can you activate the iPad without a computer? I don't know. If they can activate it in-store for you, using your Norwegian iTunes account, that would be good. You could just call or email a store to confirm.
One other thing that people might be interested in is that, upon returning to Canada, I bought a Rogers 3G microSIM to use here. However, after inserting the microSIM, I had to sync the iPad with my Mac so that it could "register" the microSIM with Rogers or iTunes...not sure which. So I'm hoping I don't have to sync every time I switch microSIMs because that would mean I have to bring my Mac with me when I travel (or pre-sync with the correct microSIM)
I seem to recall that using Starbucks wifi required me to purchase a $5 starbucks card to get a card number...then I had to create a starbucks account, which required a US address, but it did work once I had done that. My starbucks account that I use in Canada didn't work in the US.
Hope that helps!