Field test mode, missing from iOS 4.0, makes its triumphant return in iOS 4.1. If you're not familiar with it, field test mode is a diagnostic mode that shows cell reception as actual, meaningful numbers as opposed to nebulous Apple and carrier rendered bars.

To enter field test mode, dial 3001#12345#

To exit field test mode, click the home button

If you're playing around with it, say trying out death grips, death touches, or AT&T death zones, lower (as in negative) numbers are worse. And if you're playing around, let us know your results.

(Note: I received an "error performing request" on my iPhone 4 under 4.1)