And now it looks like the HD7 is on "backorder" as well (see Samsung Focus). According to TmoNews, when you try to place an order through Tmobile's website, you get the above message. A quick call to customer care results in the same message with the caveat that they don't know when it will be back in stock.
For now at least, you can still get it through Amazon Wireless, which being $50 less is a better deal anyways. Although funny thing, my HD7 order still hasn't shipped from Amazon, so who knows what's up.