David Martin in Cult of Mac on the return of Field Test in iOS 4.1:
Apple has re-introduced field test mode in iOS 4.1, which will allow you to quickly check the quality of your cellular signal by simply dialing: *3001#12345#* and pressing Call on your iPhone.
The signal bars on your iPhone will be replaced by a number. The higher the numeric portion of the negative number gets the worse your signal is. For example, -100 is worse than -79. If you tap on the displayed number the display toggles between displaying the number and the normal signal bars.