According to HTC's Official Blog the Desire HD's ICS upgrade was cancelled because:
After extensive testing, we’ve determined that the current version of HTC Sense with Android provides customers with the best experience on the HTC Desire HD. When we consider new versions of software, we weigh a number of factors, but ultimately the customer experience on the product is the deciding factor. We apologize for any confusion this change may have caused our customers.Sad ending to what was a ride hopes of an early upgrade to finding out the update was cancelled all together. For those of you who aren't satisfied with Gingerbread, and we know there are many of you, you can be certain third party solutions will be made available on XDA Developers, remember to peruse at your own risk.
Via: XDA Developers Desire HD General Forums
Source: HTC Blog