Waiting for Tmobile here
I have badgered T-Mobile about if/when they will be releasing the Z and they are stating that they will not be selling it directly. You can however buy it on T-Mobile from 3rd part vendors. The retention team have tried to pursuade me to have the HD - no thanks
I have pre-ordered mine through one of the 3rd party company on an 18 month deal at £35 a month (handset free) which works out less in the long run than the 24 month £30 a month deals. I also used a cashback site to get £30 cashback, which I should have approx 2 months into the contract.
Also, to keep my number, I have had to port my T-Mobile number to another network (pay as you go) and once the new contract starts I shall port it back.
A lot of hassle, but I get to keep my number and get a great deal on the handset I want.
Still no confirmed delivery date, but it should be next week, if I pre-ordered early enough