The HTC Desire HD was officialy announced on the 15th of september 2010 during HTC's annual launch event (link to engadget coverage). It was announced together with the Desire Z. The Desire HD have been geared with a state of the art 1 GHZ processor, a massive 4.3 Inch screen (great for watching movies, playing games, browsing the internet, or reading e-books), and 768 MB of ram. It is also packed with the latest in social networking and an 8 MP camera that is able to film in HD (hence the name?). HTC has also included their popular signature user experience called Sense, which basically makes using the phone a more personal experience. At launch the phone will run the operating system Android 2.2.
Here is a link to the HTC website which gives a more detailed overview of the phone: http://www.htc.com/www/product/desirehd/overview.html