The rep at T Mobile said that the "unlimited" plans are indeed unlimited in the amount of usage, however, when you exceed your "unlimited" tier (2gb 5gb, 10gb) then you lose speed for the rest of the month.
T Mobile has a usage calculator which I think is handy.
http://www.t-mobile.com/Tools/MBCalculator.aspx
In reality the 200 MB plan ought to be just about right for me, but on that plan if you go over then you pay overage. It's $10/mo additional to my regular plan (which is a grandfathered plan no longer available which amounts to unlimited talk for me) . For $20/mo I can get a 2gb plan which would let me play in addition to using it for on the spot research.
Of course, it wouldn't be t-mobile without a hitch. The last rep I talked to "strongly suggested" that I get insurance with any smartphone, for $8/mo which would "cover" loss, damage, theft, malfunction. I put "cover" in quotes because there is a $40 deductible. So they want you to pay $96/year for insurance on a phone that you are paying $29 for, and which REQUIRES that you carry a data plan.
So I said, "Well, if something happens to the smartphone, I'll just go back to using my Sony Ericsson." She says that I could so that, but I would still have to pay for the data plan each month, that my new 2 year contract would be for my current plan plus data.
I hate this shit. When will they stop pretending that these phones cost $500? When will they stop this contract shit? When will they stop nickel and diming us? It's as bad as AOL before cable and unlimited internet replaced Always Off Line.