Unlock BootLoader Nexus One

Nexus one was rooted before the launch (How to root Nexus One), this was possible engineering bootloader shipped on the devices distributed by Google, and that retail devices would likely have locked bootloaders. This method will let you unlock your bootloader and give users the power of Root it easily Plus install any Custom OS on Nexus One.

Apparently, Google made it easy to do it

To unlock your Nexus One’s bootloader

Step 1. Download and extract fastboot from here (Windows, Linux and Mac included)
Step 2. Run a command prompt / terminal at the directory you just extracted
Step 3. For Windows:

fastboot-windows oem unlock

For Mac OS X

./fastboot-mac oem unlock

For Linux

./fastboot-linux oem unlock

Step 4. That’s it, proceed on your merry bootloader unlocked way