Thanks for that info. You've cleared things up for a lot of people, I'm sure. I knew there were differences but not the details.

Can someone who really knows answer this question. What USA T-Mobile data plans
will apply to the N900? I'd like one that supports HSDPA (3G), not EDGE/GPRS
and allows tethering the N900 to a laptop. I'd like one that allows general
TCP/IP including web, POP3, IMAP, ssh, and tunneling X and rsync over ssh. And
I'd like one that is not throttled or proxied, one that provides a high
bandwidth low latency connection. With a data limit of at least 5G per month
and ideally unlimited (though my actual data usage will usually be about
500M-1G per month).

And I'd like one that could be coupled with a voice plan.

My goal is to select a legitimate plan consistent with the above intended
use. I need to be able to rely on the above for daily use and I don't want to
be saddled with unexpected charges or have service terminated or reduced for
violations of T&C.

http://www.t-mobile.com/shop/plans/C....mc_t=OnsiteAd

lists Total Internet at $39.99/mo giving Unlimited data and email and
webConnect Data Plan at $59.99/mo giving only 5GB data and no email. I don't
understand why the more expensive plan appears to offer less.

http://www.t-mobile.com/Templates/Po...n_Lst_DataPlan
seems contradictory in a number of ways. It disallows tethering (which would
appear to be the only mode of operation one would use for a laptop connection
card on one of the above plans) and discusses a 10GB limit instead of the 5GB
limit for the webConnect plan.

The following answer assumes that you will also buy a voice plan. I will also not touch on messaging bundles, though that should not impact the answer to "what data plans work".

The short answer is: The $24.99 smartphone plan is your safest bet for a data plan that will work long-term with the n900, or any other unlocked smartphone.

The longer answer (based primarily on discussions on this topic on Howard Forums):
1. Both the $9.99 unlimited "dumbphone" and the $24.99 unlimited "smartphone" plans have been known to work with unlocked phones.

2. As others have mentioned, the main disadvantage to the $9.99 plan is that you are somewhat limited by port restrictions, though most of the web is unrestricted. Some enterprising users have managed to get around this, but if you want things to "just work", the $24.99 plan is a safer bet

3. 3G access is not contingent on having a certain plan. I believe that debranded/unbranded phones with AWS 1700 have been used successfully on the cheaper plan, with 3G speeds. You just need a phone with the requisite bands, and 3G coverage in your area.

4. These plans are billed as "unlimited", but the fine print in the TOS states that T-mobile can throttle down your data speeds if they view your usage as excessive. In practice, this appears to be rarely enforced--probably because there aren't that many devices out there that can tax the network (like the n900 probably would)

5. T-mobile has really tried to clamp down on tethering, but if your phone has that function, they really can't stop you other than throttling your data due to excessive use

6. The $19.99 Total Internet plan (which I use right now with an unlocked phone + n800 combination) also should work, and is a great value since the TOS does not explicitly prohibit tethering, does not mention a data-cap (though admittedly, probably will not stop T-mobile from doing something if they believe usage is too high) and has unlimited hot-spot access. But, I believe this is no longer offered to new customers.

7. The other watch out for using the $9.99 plan which has come up recently is that it will probably eventually stop working if T-mobile decides to subsidize a version of the device. Generally, T-mobile doesn't care if you have an unlocked phone (there is a debate in the Howard Forums community as to whether T-mobile is unable to detect the IMEI codes of unlocked phones, or whether they can but just don't care). But, in the experience of the purchasers of unlocked Android phones that were later picked up by T-mobile: the $9.99 plan (and even the $5.99 plan) worked perfectly for awhile on these unlocked devices, but as the release dates of the T-mobile branded versions of these phones drew closer, the owners of the unlocked versions saw their data shut off. T-mobile won't do anything drastic to you (like remove you as a customer) in addition to that, they will simply send you a message to upgrade to the $24.99 plan if you want to regain access to data. You should be good to go after that, without any repercussions.