My N900 is working fine with the $10 t-mo (US) web plan that I originally ordered for an SE TM506. I let the N900 auto-configure thinking that I'd have to tweak the configuration later, but it works just fine. I SSH over it on a semi-regular basis with completely tolerable amounts of latency. I used to tether my TM506 to my N800 and stream last.fm. Worked pretty well as long as I had a solid 3G connection. Overall I'm sure that I don't use more than 200MB a month, though.
As for the people hoping t-mo will subsidize the N900; wouldn't it be cheaper in the long run to just by the N900 on a credit card and mix-and-match the plan that you want from t-mo (the $10 web plan + however many minutes you want)?

-John