After iOS 4.1 was released on Tuesday of this week, reports started to come in that the new firmware has restored the ability to get into Field Test mode on iPhone.
For those wondering, Field Test mode allows users to get a numerical display for the antenna strength instead of the current signal bars. This allows device testers to properly gauge signal strength.
To get into Field Test mode on your iOS 4.1 iPhone, do the following:
1.) On your Home Screen, tap Phone
2.) In the Phone app, tap on Keypad and dial the following:
Then tap Call.
3.) To exit, press the Home Button.
As a quick note, more negative numbers indicate worse signal strength.