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