Worked for me. Although the force checkin step did not work, stopping google services and clearing the data worked on both my Nexus S 4g and my Galaxy Nexus. :good:
Finally got my Nexus to start downloading Jelly Bean after 3 days of nonstop tapping the check now. Follow these steps an you will be using the buttery goodness like me.
-Go into Setting -> Apps -> All -> Google Services Framework
-You will see a button for “Clear Data”; clear it.
-Go into your dialer and type *#*#2432546#*#* - This will cause a ‘force checkin’. You will notice that it shows “checkin succeeded” across the top.
Go back to Setting -> About Phone -> System updates and then click the check now button.
YOU HAVE TO BE ON WIFI TO DOWNLOAD UPDATE!!!
Hit the thanks button if it helps!!!! Best of luck!