How come I can't sing?

This thread made me want to learn more about tone deafness; unfortunately, there doesn’t seem to be a whole lot of good information on the web. I did find this one site that seemed intresting:

http://ccrma-www.stanford.edu/~levitin/TD_casys.html

It doesn’t have any answers, but it gives a very detailed accounting for the variety of problems that people might come to call tone deafness.

I was especially interested in the concept of “total word deafness”, which seems to imply a complete inability to make sense of certain kinds of sounds–for example, an inability to distinguish people’s speaking voices–even though they can “hear” them just fine.

If anyone out there knows of a good source that discusses tone deafness from a broad and informative perspective, I would be interested to hear. Thanks.