I looked up iMic on Amazon and read all the reviews. This is the URL:
http://www.amazon.com/dp/B00006BALQ/?tag=macforums0e4-20
The answer may lie within. The product description (or perhaps it was one of the reviews) suggests that there is a LEVELS difference between a Mic In port and a Line In port. Unpowered microphones produce a very low level of signal, whereas a powered Line Out from say a stereo that you connect a Line In somewhere else produces a higher level (higher power basically) signal. Hence, Mic In ports are set up to detect and deal with much lower power signals than Line In ports.
What you have on your Mac these days is a Line In connector, not a Mic In. As a result, your microphone is probably being seen by the Mac, but the audio level is so low that nothing much is being picked up.
This jives with my experience. One day, out of sheer frustration, while I was in the Audio preferences panel, I literally started yelling into the microphone, with it right up to my mouth. I actually could see the very first bar of the Preference panel signal strength meter light up now and then. I should have realized right there what the issue was. This also jives with the performance of these mics under Linux. Under most standard Linux audio drivers, there is a "20 db gain" checkbox for Mic In, which I have always checked for microphones. If I don't check the box, I can't record from the microphone.
So, deductive reasoning plus some reading leads to the conclusion that the Line In input on your Mac requires a powered microphone, or something like iMic, which includes a small pre-amp in it.
Problem solved?