HTC has only just showed off the Desire Z's fast booting and no sooner has it showed us all its cool new toys but the Desire HD ROM was leaked. It was only a matter of time before CyanogenMod got it's hands on it and start dissecting the ROM and taking some of the nice bits.
HTC achieves the fast boot by basically dumping all of the operational data into the RAM. It puts the phone into a hibernation state, this makes it boot really fast.
The video below is showing an early implementation of the CM fast boot in action in build 6.1 on a Nexus One. We probably won't see this release for a little while but it certainly gives his user base something to look forward to. Cyanogen is announcing support for more and more devices so keep up to date to see if your device will be added. My guess is that this feature will only be added to devices with the appropriate amount of RAM.
Check out the video: