The retail units have locked bootloaders however Paul at Modaco forums has managed to unlock it so that you can root your retail Nexus One for full file system access on Android 2.1.
You can follow the step by step guide posted below (on your own risk) to unlock the bootloader on the Google Nexus One phone. We are not to be held responsible if you ended up bricking your phone. The steps are as follows.
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: Type ‘fastboot-windows oem unlock’ or ‘./fastboot-mac oem unlock’ or ‘./fastboot-linux oem unlock’ (as appropriate).
Step 4: Proceed on your merry bootloader unlocked way.