1. Do you need to change your existing plan to use it? I am still on old GRPS grandfather plan?
2. Did TMo disable the internet sharing function? I don't know where I read that but best to ask those users who already have them.
I think the internet sharing program was on the stock ROM (I didn't stay stock very long), but it is definitely on all the custom ROMs.
Also, If you buy the phone out of contract, I'm pretty sure you can use your existing plan. They might catch on after a while though, this thing chews through data.