Data plans are not required for unlocked phones. The N900 only works with their EDGE service, and so long as you don't walk in with the N900 you can tack on their cheaper "unlimited" MediaNet plan (I'm paying $15/mo) and they won't know or care.

When I got my N900, I just swapped the SIM. Worked exactly as I expected. Better, even, as it had the APN data already.