Unlocking the Bootloader on a Nexus One with S-On
Nexus Ones do not ship with 'S-OFF', they ship with a "locked" bootloader which then must be unlocked. You can actually use fastboot to unlock the bootloader, just run fastboot oem unlock on your PC using ADB (Android Device Bridge). MoDaCo has a post here with complete instructions and a multi-platform download of the needed tools.