My situation is similar to yours. Before I got the N900, I used an unlocked phone with a T-mobile US prepaid SIM and used a N800 with wifi when I wasn't near a laptop.

The T-mobile prepaid plan is a great way to buy small amounts of voice airtime and SMS and their prepaid SIMs should work in any unlocked GSM phone, including the N900. Once you spend $100 on the account, any refills are good for a year so this is much less expensive than most prepaid plans for very light use. I got my SIM many years ago bundled with a very cheap backup phone at a T-mobile store because I was in a hurry at the time, but a bare SIM is even better for you since you already have another phone.

Data is a bit more complicated. The only smartphone data plan that a T-mobile service rep will tell you about is their Even More Plus plan with 500 monthly voice minutes and "unlimited" data and SMS for $60 per month or $80 with unlimited voice. The plan is month to month with a $35 fee to start and no charge to stop. I haven't tried it but it looks like a good deal for someone who expects to use it heavily.

At the other extreme, T-mobile provides limited web connectivity to a few sites for free on prepaid accounts. I haven't bothered trying it on my N900, but it worked on my old dumbphone. If you search the forum, you will find discussions of other inexpensive data options through T-mobile and ways to get more out of them than the carrier expects but I believe that all of them violate the terms of the subscriber agreement in some way. Lots of iPhone owners used to sign up for the Sidekick plan but T-mobile has cracked down on that practice, for example.

The US carriers are really overpricing their data services by only making them available on top of ridiculously expensive voice plans that few people really need. I have been tempted to just pay the $60 per month for a Mifi that I can use with my N900, N800, laptops, etc and be done with it but I don't really want yet another gadget and wifi is hard on battery life.

Now that the phony "unlimited" plans are starting to go away, the US mobile data market may start offering us some better choices that don't involve a contract or a bundled voice service. I definitely wouldn't submit to a two year contract today because things are changing fast.