Test your iPhone’s antenna with iOS 4.1’s Field Test

"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," Martin reports. "The higher the numeric portion of the negative number gets the worse your signal is. For example, -100 is worse than -79."

Martin reports, "If you tap on the displayed number the display toggles between displaying the number and the normal signal bars."