To put your iPhone in field test mode, so that you can see the actual signal strength numerically:

Open the phone application.

Enter the following from the keypad: *3001#12345#*

Press call.

This will put in field test mode, and will replace your bar signal strength with the numerical representation.

Evidently, this has been blocked in a recent version of IOS. I did this from a jailbroken IOS version 4.1 (8B117) with baseband 01.59.00.