Check out the Music and Neuroimaging Laboratory at Beth Israel Deaconess and Harvard Medical School. Their list of projects is particularly interesting. And I took their tone-deafness test! Here are my results. I wish I knew how I stacked up to other people, but they don't tell you that! Also, they don't tell you how many you got wrong, or how small the pitch differences in the test get.
"Our results show that, given a baseline pitch at 500 Hertz (Hz), you can reliably discern pitch differences of less than 12 Hz, which is a small interval considering the fact that the semitone, the smallest interval used in Western music, is about 30 Hz (6%) with a 500 Hz baseline. Therefore, you are NOT tone deaf. The majority of the population can discern intervals smaller than the semitone, but this auditory acuity varies greatly and may be improved by substantial musical training."
I have probably linked to this before, but it's cute, so why not again? (Don't be thrown by the fact that the French apparently don't distinguish between upper and lower case Roman numerals for major and minor chords.)