Using an unlocked 3g iPhone iOS4 with an AT&T SIM

Hey all, I've searched around but, I can't seem to find a direct answer to my question.

I currently live in Shanghai, China. However, I frequently travel back home to the USA, and visit family in Holland often. I use unlocked phones to avoid the headaches associated with travel. I usually buy a prepaid SIM in whichever country I'm in.

I'm looking to purchase my friend's old 3g iOS4 since he bought a new 4g iPhone. After unlocking the phone, will there be any sort of problem when if I plop a prepaid AT&T SIM card in it for my month in America?

Thanks, Cody

Nope! You will be all set to go...as long as it's a proper unlock. My cousin did te same exact thing with the older gen iPhone when we went to europe.

wow you get to travel like that.. what do you do and do you have any job openings. i'll live in china..

@macwannabe: I'm actually a student, going to international school in Shanghai, with half the fam in Atlanta and the other half in Den Haag.
Edit: Also I work partime-ish for citymoments.cn (photography)

@idrinorbarsku: Thank you, I assume Pwnage tools will do the trick?

I can't give any advice in regards to pwnage tools because We actually just took the iPhone to an unlock store right next to the Att corporate store. It had a big "Unlock your iPhone here!" sign, so we gave it a try and it was perfect. It took literally less than 5 minutes, and allows you to put any sim card, through any carrier, and can also be used on pre-paid cards...as long as you have their sim card.

I'm on AT&T in the US. My 3GS happens to be unlocked via Pwnage/ultrasnow and it works perfectly.