From what I've seen there are 3 options when you want to downgrade Hboot:
sw 2.10<: http://forum.xda-developers.com/showthread.php?t=1399331 (no need for htcdev.com)
sw 2.11>: http://forum.xda-developers.com/showthread.php?t=1443636 (no need for htcdev.com)
Latest official ICS: http://forum.xda-developers.com/showthread.php?t=1318919 (need htcdev.com)
sw 2.10<: http://forum.xda-developers.com/showthread.php?t=1399331 (no need for htcdev.com)
sw 2.11>: http://forum.xda-developers.com/showthread.php?t=1443636 (no need for htcdev.com)
Latest official ICS: http://forum.xda-developers.com/showthread.php?t=1318919 (need htcdev.com)