rooting - Root N1 and keep the official ROM

Is it possible root my N1 with 2.3.3 without flashing custom ROM?

Answer

Yes. According to this XDA thread, you can unlock the bootloader with fastboot and then flash only a custom recovery. The custom recovery will allow you to put su on your device.

You could also flash a stock 2.3.3 ROM that has been altered slightly (i.e., pre-rooted). For example, this one over at XDA.