I have done reverse (US account with Canadian IP) and the account went down right away. There might be ways to do this - maybe even letting pp know that you are doing work in the US - but I would wait for other folx who have experience to chime in.
A few users come to mind but I wont point them out - they will post if they feel like =)
What I did was I was at home on a Canadian IP. I registered a pp to an address in the USA. It got limited RIGHT away after creation - prob because of IP/address mismatch.
I have done reverse (US account with Canadian IP) and the account went down right away.
You probably are doing it wrong, assuming the account was originally registered in a native US IP. I have done and heard from others about doing both ways, both are fine as long as you establish plenty of browsing and small buying activities before doing anything major.