HTC has been mum about Windows Phone 7. I guess their concentration is on HTC Desire HD and Z. Nevertheless, AT&T is using one of their devices in their adverts. It is the HTC Mondrian.
Nowadays, it is more difficult to identify Windows Phone 7. They looked the same, thanks to Microsoft for all the rules. Here is HTC Mondrian shown on AT&T advertisement. Enjoy before the real thing arrive.
and here is another one.
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(source)