Fast Boot Restart
I had previously missed mention of HTC's "fast-boot" functionality which is part of the HTC Sense customisations. From power-off it gets the device ready to make a call in about six seconds (A regular start will take upwards of a minute). From what information is out there it's an ultra-low-power sleep state - something in the PC world we'd call S3 or Suspend-to-RAM. Cyanogen has already replicated it in the 6.1 series.
Without more information it's difficult to be certain how this functions technically but my educated guess is this: When selecting power-off in the user interface the Sense/kernel sends a particular oem-XX code or calls an RPC function to tell the radio to shutdown and remove power from the ARM11 applications processor. I'd suspect the ARM9 radio continues running REX/AMSS but turns off power to almost everything. When the power button is pressed REX brings up the power to devices and restarts the ARM11. Now, the question is, did the ARM11 register state get saved by REX/AMSS so it can be restored and execution restarted where it left off, or does it restart via hboot. If it goes the hboot route then hboot will detect it's a fast-boot and restore contexts and then jump back into the kernel where it'll unfreeze itself.
Update: Having tested this on a U.K. HTC Desire Z it looks likely that hboot isn't re-executed. The green-on-white "htc" splash screen appears briefly (1/2 second maybe) then the Android lock-screen appears. The time take to be ready is less than two seconds. Holding down key-combinations that usually cause hboot to enter special modes are not recognised.
Sources:
New HTC Fast Boot Technology Already Present On Desire Z/HD Offers 5 Second Boot Times. Smarts, Trickery, Or Both?
Cyanogen Demos HTC Fast-Boot-A-Like On Nexus One
