On the previous iPhone OS versions there was a way to open Network Monitor, or so called Field Test. By pressing *3001#12345#* and <Call> you could activate a screen similar to the one on the right. Note, that the usual “bars” are replaced with dB indicator showing the signal strength.
Unfortunately the above code only works until iPhone OS 3.1.3. In iOS 4.0 there is no such thing. Sad Panda Face again…