Yesterday, I start trying to root my Nexus one, but I had my bootloader locked, I couldn't root it from Gingerbread version, so I had to restore it to a previous version. If you want to restore a older version use this steps:
PS: BE CAREFUL WITH THE ROM THAT YOU CHOOSE. THERE ARE SOME ROMS THAT JUST WORK IN SOME DEVICES. IT IS IMPOSSIBLE TO RESTORE IT DIRECTLY TO FRG83 USING PASSIMG.ZIP METHOD!
1. Download the ROM version that you want from XDA Nexus One ROMs
2. Rename the zip to PASSIMG.ZIP
3. Copy the PASSIMG.ZIP to your SD card. (Without using any folder.)
4. Turn off the phone
5. Turn on the phone with Volume down button pressed
6. Your phone will show a screen looking for files (WAIT, it could take up to 3 minutes)
7. The phone will ask if you want to start the updates. Press Volume UP button to start it.
8. Your system will start with the ROM that you choosed.
REMEMBER! Be careful with the ROM version