Ebay has a deal going on right now to unlock ANY HTC DEVICE for $2.50. You can find the link here.

You dont need to revert back to winmo to unlock your device. Once you get the code you will need access to a AT&T sim card (or other GSM sim card) so you can complete the process though. Just insert the other sim card and punch in the code given and thats it.

EDIT: This is not valid anymore since he/she has changed the price to $4.99. See below for a different deal

I know this has been answer to death but i did want to point out a seller on ebay who has any HTC phone unlocked for $2.99 obo.
That is if you've already called T-Mobile and they wont unlock it for what ever reason. I had unlocked a few phones from them and
all i did was call up and say i want to sell my phone for a little more so can you give me the unlock code. It was that simple. They gave
me the code within 2-3 mins. Here is the seller i was talking about on ebay. I would contact him/her to make sure but it says HD2 in the title.

does the Tmobile HD2 work on att if unlocked ?

It will work but if i remember correctly either 4g or 3g speeds are not compatible. This has not stopped the many people who have unlocked their phone and used it with At&t though.

If you have an active account with T-mobile you can call up to get a code to sim-unlock your phone so that your phone can be used on At&t. If not then you will need to buy an unlock code from the internet.

Or just call T-Mobile for your code... like I did...

Did you call T-mobile for the unlock code? Otherwise you can also get the unlock code at Mobile-Unlocker.com very easily. They provide very reliable service.

You can unlock your mobile by using the unlock code .There are many online service provider for getting unlock code but you have to choose the best one . I suggest you to visit the site Superunlockcodes.com . Here , they ask to provide the model , country , network provider name and IMEI number of your mobile . After they deliver the unlock code for your HTC HD2 mobile through mail . After unlocking , you can use any use GSM network on your mobile .