In older versions of the iPhone OS software, it was possible to enter Field Test mode to get a lot of useful data about the cellular connection. This capability was removed in iOS 4.0, and caused some angst while trying to examine the iPhone 4 antenna reception issues.
The ability has been restored, although to a greatly limited degree, with iOS 4.1. Once again, if you dial the number *3001#12345#* in the keypad you will enter FTM. What you will see now is a negative number (presumably representing the signal loss in dB) in the upper left that replaces the cellular signal strength bars. A more negative number means lower relative signal strength, so the -121 I have at the moment is worse than the -92 I had in a different room in the house. Zero would be no signal loss.
To exit Field Test mode, just press the Home button. It may take a moment for the number to go away and be replaced by the bars again.
I think we all hope that when iOS 4.2 is released (Apple says in November) it will restore full Field Test mode with all the diagnostic data available, and maybe bring it to the 3G enabled iPad as well.
Mac OS X Hints
http://hints.macworld.com/article.php?story=20100910053612812