I am not sure such a case exists for the HTC Desire.
All of the LEGO style phone accessories I have seen in the past have been for the iphone.
A bit of searching turned up these items:
What appears to be a case with LEGO compatible studs (for iphone)
Check out this page for other LEGO relates accessories and a picture of the above case in use with actual LEGO built onto it (near the bottom of the page).
There is also a TinkerBrick LEGO iPod/iPhone 3 case which has studs on the front and sockets on the back. There are taking Votes for the next supported phone on their site.
There is a mock LEGO silicone iphone case available here. The two pieces can be attached in different positions.
To wander off topic slightly, there are other 'phone related' LEGO style accessories available that are compatible with most phones:
Dock cover available here.
LEGO brick earphones available here.