iOS 4.1 Brings Field Test Mode Back to the iPhone
Users are reporting that iOS 4.1 once again allows you to enter Field Test mode on your iPhone and get a numeric representation of signal strength.
To try this you must first update your iPhone to iOS 4.1 then dial *3001#12345#*.
You'll notice that your signal bars are replaced with numbers. The closer the number to 0 then higher your signal strength. For example -10 is much stronger then -100.