T-Mobile started selling HTC HD2 phones based on Windows Mobile 6.5 in stores earlier today, however, it looks like the phone was so popular that T-Mobile retail stores have run out of stock in the first few hours itself.
I came across several forum users who said that T-Mobile stores were sold out as early as 9:30AM in some places. Others were able to purchase the phone from Walmart, however, more and more users are returning empty handed from the T-Mobile stores.
TMO News also came across sources who said that the HTC HD2 were sold out across most of the retail stores. However, some stores still have the device in stock, so you might want to rush to the store before they sell out too.
Everything aside, if you do not get your hands on the HTC HD2, you can always order it online at the T-Mobile store and pick it up in-store after they have restocked in a day or two.