From The Apple Wiki

The Field Test application, normally hidden from the SpringBoard through a property in the Info.plist file, can be accessed by opening the Phone app and attempting to call the following number: *3001#12345#*

In iPhone OS 3.1.3 and earlier, the app displayed a wide array of information about the GSM/UMTS network. This application was not available in iOS 4.0.x; it returned in 4.1 with the sole purpose of temporarily enabling the RSSI meter (a feature also available in previous versions of the app and 4.0.x by editing SpringBoard's preferences.)