After my wife’s Nexus S was updated to Jelly Beans more than 24 hours ago, I could not wait any single second longer. Looks like there is a way to manually trigger the OTA update:

• Search for Google Framework Services in your app manager and clear all data.
• Afterward put the phone into flight mode to disable any gsm/umts connection.
• Now just re-emable Wifi (Widgetsoid allows this).
• Finally let the phone check for new system updates and install 4.1.1.

For me this worked pretty well on first try and my Nexus S is on Jelly Beans now, but I read that it may be required to repeat these steps a few times.