Hello there! As of 8-27-2012 I have been chosen to update this thread as Drakknar no longer
owns this device. Without further ado, here are the latest radios for Desire HD. I will
try my best and keep you up-to-date with everything that's going on with the scene.

HOW TO UPGRADE - Bootloader Method - Considered the Easiest
Requirements: S-OFF
- Download the radio file
- Rename it to PD98IMG.zip (this is important!)
- Move the file to your SD-card root (not in any folder)
- Download MD5 Checksum Checker: http://download.cnet.com/MD5-Checker...-10410639.html
- Make sure that the MD5 provided with the download link matches with your downloaded file!
^ If not performed, you might risk bricking your device!
- If they match, you are ready to go! Power off your device.
- Hold the volume-down and powerbutton simultaneously
^ This will boot you into Fastboot/Bootloader
- You should see a screen with options
- Navigate to Bootloader (if not already in it) and press the powerbutton
- Wait for the flashing process to begin (it does automatically!).
- Reboot your phone and you are done!

July 2012: Radio - AT&T Inspire 4G 3.20.502.52 (Latest)
Radio.img MD5 Checksum: fefbcf7a411df7df31fea65417d92f36
Mirror: http://fbe.am/b95.
RIL: http://d-h.st/Kq4 [**]
Feedback (Northern Europe): Better reception indoors than with the 26.16/26.14, HSDPA
speeds work and display correctly. Download speeds are a little low compared to upload speeds
but can be fixed with this application. Used so far: approximately 5 months.

Please note: As it seems, this downgrading from this radio might be bugged.

[*]: Recommended download
[**]: Still untested, be careful before flashing

If you are willing to mirror these files, please make sure the MD5s match!
Unfortunately not all of us seem to have a habit of verifying the checksums..
Also remember to provide checksums when posting download links!

Refer to the original post.

Thanks to JSLEnterprises for pointing me to Radio download.