Field test mode has returned to iOS
4.1. That means that those of you who want to see just how bad your iPhone
is, in real terms (dBm), as opposed to signal bars, can.
Assuming you have iOS 4.1, all you need to is type *3001#12345#* on your phone, then press dial. You'll see a negative number. The higher the absolute value of that number is, the worse the signal strength is. For example, a measurement of -110 dBm is worse than -85 dBm.
To exit field test mode, you can simply hit the Home button.
Field test mode had earlier been removed from iOS 4. Anandtech found a way around that, but only with a jailbroken iPhone (by using a 3GS image), whne they conducted a series of test during the heart of the "death grip" controversy surrounding the iPhone 4.
Readers, do you see the signal strength drop (absolute value of the dBm reading rise) when you touch the appropriate death grip spot bridging the antennas on the lower LH side of the iPhone 4 (bare, of course, no case). How much of a drop do you see, if so?