T-Mobile usually has the best plans, but they're not THAT good. That plan shown in the link is for certain phones only, in this case the T-Mo Shadow (look toward the left above the price). Also, notice the active tab above is 'Other phones' and there are other tabs for different models. The 'Smartphone' tab is the one that would apply to an unlocked N900.

Currently I think the only choices are $25 for unlimited data, no messages included, or $35 for unlimited data + unlimited messages. I'm on a $25 unlimited data + 400 message plan, but I'm grandfathered in, I don't think it's available for new users. And by 'unlimited' I hear (though I can't prove it) that T-Mo doesn't get nervous 'til you cross the 5GB usage line. That's pretty hard to do with a phone, I've never come anywhere near that with my G1. So, yeah, it's basically, though not totally, 'unlimited. Close enough, I guess.

Still, I've been with T-Mo about 12 years and always gotten as good or better a deal as anyone, never had a messed up bill, always good service. No plans to change.