Just tell them you'll be using it with "my Nokia phone". For most stores, the conversation will end there, as they will assume you have a dumb phone. If the sales person is semi-smart, they may ask if it's an N-guage, which of course the N900 is not. :P

In any case, enjoy T-Mobil while you can. If the merger goes trough as planned with AT&T, this time next year you won't have the option of using anything nearly this cost effective in the US. (And that's coming from a current US AT&T customer...)