For those with a nexus s looking for Jelly bean update you can force it using this method.
Worked for me anyway.
You can force OTA update to be installed immediately using the following steps:
1) Go to the System Settings->Apps then hit the ALL tab at the top panel (swipe left if you don't see it)
2) Scroll down to Goggle Service Framework and open it
3) Hit the clear button and confirm
4) Go to the System Settings->About phone->System updates. The last checked date will be 1969 or 1970, this is ok
5) Hit the check button
6) Nexus S should now show update to 4.1.1 is Available
7) Hit install and enjoy