WARNING: This will wipe everything on your phone
-Clockwork recovery or other custom recovery
-The nandroid backup from here http://www.mediafire.com/?ontvvh8z9h9lb7a
-Transfer cable, to transfer contents of downloaded file to phone's internal memory
1. Download nandroid file from link provided above
2. Extract the .zip file on your computer
3. Move the folder "2010-12-22.21.06.07" to the clockworkmod/backup/ directory of your phones internal storage
Note: if you have never used nandroid you wont have the directory clockworkmod/backup on your phones internal storage, so you will have to manually create those folders, then put the folder "2010-12-22.21.06.07" in the new folders you created
4. Now go into your clockwork recovery manager, you can do this by typing in "adb reboot recovery" if you have adb setup on your computer, and the phone is connected. Or while the phone is turned off hold the volume up+power to get into bootloader mode, then go down to "recovery" by using the volume up/volume down keys to scroll up or down, then press the power button to launch it, and this will launch the recovery manager
5. Once your in clockwork mod recovery select "backup and restore"
6. Then click "restore"
7. Now you should be able to choose and select "2010-12-22.21.06.07"
8. Clockwork will now restore this nandroid backup
9. Reboot system
Note: This version of android build "GRH78" will make sure you have the stock recovery manager everytime you reboot the phone. So after doing this, after you reboot you will no longer have clockwork mod.
Edit: According to xda member Luxferro the new update introduced /etc/install-recovery which may have not been in 2.3, but anyways, it runs this on bootup, and if the MD5 of the recovery doesn't match what is expected, it reflashes the stock recovery back over, so if you dont want the stock recovery to be flashed everytime you boot if your on clockwork or another non stock rom, just rename it to prevent it from running on boot. And for those of you who want to return your phone back to stock this nandroid will do that for you since it also flashes the stock recovery.
Video on how to unroot the nexus s and return to stock
Video via xda member dcogen