Windows Phone 7.8 is said to offer a semblance of what Windows Phone 8 is all about for Windows Phone-powered devices that will not be getting the Windows Phone 8 upgrade, and the venerable HTC HD7 is privileged to receive the Windows Phone 7.8 Beta ROM recently, making it the very first Windows Phone 7.8 Beta ROM to boot. This particular ROM release is known by its build number of 7.10.8835.35, although it has its roots from a Tango 12180 dump from a Nokia device. There was minimal testing done before the Beta ROM was released, so you can be sure that this is far from the finished article.
Do remember to proceed at your own risk if you are interested, and be prepared for a very raw Windows Phone 7.8 experience since there will be no support for homebrew apps, although you can always opt to re-flash your HTC HD7 with a Windows Phone 7.5 ROM at a later period. We did catch a leaked ROM of Windows Phone 7.8 on the Lumia 800 this month as well just in case you own the Nokia device.