One of the new overlooked items in the iOS 4.1 update is Apple's Field Test mode. This is a diagnostic mode that shows your actual cell reception in numbers rather then the larger (thanks to iOS v4.01) mini bars in the upper left hand corner.
To test it out, go to the call screen and dial:
*3001#12345#*
Once you've got enough, just click the home button and your back to the normal home screen.
Source: TiPB.com
