Added: 6 Jan. 2013
Since I originally wrote this guide, a number of CIDs not listed here have received OTA updates from their provider that also have the new hboot. If you are one of these users you now probably have hboot version 1.36, you can go ahead and flash ARHD JB ROMs.
Prerequisites: Have these files ready, if your CID is good, you will need them.
Unlock_code.bin <- You got this from HTC Dev when you first unlocked your device [If you lost it, you can get it again from HTC Dev using the same credentials you originally used or do the unlock process again from the beginning on HTC Dev.]
firmware.zip (hboot 1.28)
ARHD 11.1 ROM <- Copy this to your SD card
cwm recovery 5.8.4.0
{For v14.1, replace the following files listed above with these ones:
ARHD 14.1 ROM
firmware.zip (hboot 1.31)
}
Go into fastboot screen and check your current hboot version. If (
and only if) you have hboot version 0.43, you need an additional step:
Others please continue to step 1.
{
Do the following only if you have hboot version 0.43.
Download this ROM. You need to flash this RUU after you relock but before you flash firmware. Your hboot version will say 1.12}
1)
Before you start, make sure your CID is on the list, if not
DO NOT ATTEMPT TO FLASH!
How to learn your CID:
fastboot oem readcid
If your CID is any one of the below go to step 2, otherwise forget about JB until further notice.
Added 3.12.2012: (Since I wrote the original post, some CIDs not listed have received OTAs (which includes the new hboot). If your hboot has been upgraded to 1.3x by an OTA, you may go ahead and flash ARHD JB ROM)
cidnum: HTC__001
cidnum: HTC__E11
cidnum: HTC__203
cidnum: HTC__102
cidnum: HTC__405
cidnum: HTC__Y13
cidnum: HTC__A07
cidnum: HTC__304
cidnum: HTC__M27
cidnum: HTC__032
cidnum: HTC__016
cidnum: HTC__J15
Your CID has to be HTC_xxx. Please don't even ask about O2_xxx, VODAxxx, TMOBxxx, only HTC_xxx works.
2) Make sure you have your Unlock_code.bin file.
Relock your device using fastboot:
fastboot oem lock
Enter OEM flash mode using fastboot:
fastboot oem rebootRUU
On your phone you will see a black screen with silver HTC written in the middle
Flash firmware:
fastboot flash zip firmware.zip
(You need to do this immediately twice)
When you see a full green bar below, you are done.
3) Manually boot into fastboot on your device. Power off -> Power+ volume down
4) Unlock device:
fastboot flash unlocktoken Unlock_code.bin
5) Flash cwm recovery:
fastboot flash recovery recovery-clockwork-touch-5.8.4.0-endeavoru.img
6) fastboot erase cache <- Maybe not required but certainly doesn't do any harm
7) Get the boot.img from inside the ROM and flash:
fastboot flash boot boot.img
8) Enter recovery and flash ROM
This is as baby proof as I can get it

Enjoy!