If your iPhone is still in lower OS version, you should upgrade the firmware to IOS 4.2.1 now to enjoy the field test mode again.
In IOS 4.0 no Field Test Mode was given. In IOS 4.1, only the signal bar changed to signal level without any detail information. Now ISO 4.2.1 give all the detail as before with the signal level indicator.
The latest version (Nov 2010) is 4.2.1 (8C148). Some screenshots are attached.
p.s. to activate field test mode of iPhone, type *3001#12345#* and hit call