If it's T-Mob it'll be capped at 500Mb @ 350Kbs regardles of what they say.
Pop over to teh T-mob forums and you shoudl be able to get this confirmed, I would eat my hat if they say in writting that it is unlimited.
It is unlimited, however at 500MB they will block streaming and downloading. Web browsing is unaffected and you can continue to surf. No one is really clear how this works. E.g. if they just allow http/https traffic and block anything else, how they stop you downloading using http, etc. so it's unlimited but it's not really. You will be limited to 3G speed though, as you say, and as no one really makes the distinction between 3G and 3G+, it causes a lot of issues with people claiming they get slow speeds.
Personally, I'd avoid T-Mobile unless you buy direct and get their booster package to get full HSDPA/3G+ speeds. Buying direct may get you this included, buying from a third party won't. It used to be called Web and Walk Plus, not sure what they say it is now.