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
