Hi, just reporting in. I took my 2 day old Desire HD back to the shop for service today.
It was blacking out like you all have described. All i could do was to pull the battery out.
Heres some info i think i should share that i didn't see here on this thread
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Disclaimer: I didn't read all 78 Pages and I didn't read this thread before i gave the phone back for servicing so i have not tried the workarounds you have posted
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-My Desire HD was blacking out almost immediately after i put in my PIN just like bigles described above. I could see the white HTC boot screens, and also i was able to type in my PIN. When the desktop was loaded it took anything from 2 seconds to 2 minutes to the screen to black out.
-My phone acted 100% normal when i removed the SIM card, so i think the sensor has something to do with the ability to make/recieve calls.
-I'm not sure why the sensor decided to black out the screen even when i was not on a call.
Any way, Bigles should try removing the sim card to be able to install the calibration app. Unfortunately I wasn't aware of it before I took my phone for service. I'm actually having second thoughts on getting the phone back to myself from the service
my 2cents,
Miikkahoo