You've just read or heard that your phone is due for an update—maybe just bug fixes, or possibly a big version upgrade. But your phone claims there's nothing doing. Dial this keypad combination to force a "checkin" and possible upgrade.
If there's no update available for your phone from your carrier, then this trick won't magically compile and distribute one, of course. But there can be a lag between when a carrier makes an update available and when your phone checks in to see if it's available—even if you hit the Settings menu, choose About phone, and click the "Check for system updates" option. Sometimes it works, and sometimes it doesn't—but it doesn't hurt.
To get your phone into the queue right away, dial *#*#checkin#*#*, substituting the dial pad numbers for the letters. I used it on a Nexus S to pick up a bug-fixing upgrade, and users of other Android phones on different networks, like the Droid Eris on Verizon, have seen some success with it. If you've had success with this trick, or felt like you were a lab rat pushing a lever, let us know either way in the comments.
Nexus S gets its first update over the air [Engadget]