Your 3GS must be running OS 3 or earlier. Once you upgrade to OS 4, you can't enter Field Test Mode. (I misspoke earlier--Field Test Mode wasn't removed from the iPhone 4, it was removed from iOS 4.)

If you are still running OS 3 or earlier, go to your phone dialpad and enter the following sequence:

*3001#12345#*

(You may need to hit "call" after entering that sequence; I can't remember.)

There's lots of technical geekery in Field Test Mode that is fun to look at but makes little sense to the untrained eye. Basically, you can "see" the various cell towers nearby that your phone can see along with some other information about the cellular network. (Field Test Mode was designed for AT&T field test employees to run diagnostics on the network from, well, the field.)

I guess Apple decided that the information that Field Test Mode gives about the cell network was too great a security (or PR?) risk to chance having even a revamped entry code leak, so they removed it altogether, according to a recent blog comment by someone who is supposedly knowledgeable about such matters. On the other hand, they did leave in the code required to display the signal strength meter in dB, so maybe they did just obfuscate it rather than delete it completely...