A few years ago I was listening to Malcolm Gladwell's Blink on CD, and in a section about taste, he casually mentioned that the taste of breastmilk was "umami."
There are five basic tastes, according to people who know about these things, and they are: sweet, bitter, sour, salty, and umami.
We know the first four, but umami (a Japanese word) is a less familiar concept. It means "meaty" or "savory," and is used quite a bit in Asian cooking. Foods in this category include parmesean cheese, fish, cured meats, ripe tomatoes, and fermented products like miso and soy sauce. It serves the function of "rounding out" other tastes. Kind of like you wouldn't sit down and eat a chunk of parmesean cheese, but a little of it makes lots of dishes taste great.
Now I knew that human milk is sweet - if I remember correctly it's many times sweeter than cow's milk - but is it really also "meaty?"
The substances which give umami foods their flavor are glutamates and neucleotides, and breastmilk contains as much as soup broths. Ten times more, it's reported, than cow's milk. It's all in this chart from the Umami Information Center.
And even more interesting, umami is a flavor of amniotic fluid (which I must say is largely the baby's urine), though at lower levels.
I think that's about all I have to say about this, but be sure to check out this set of photos of babies reacting to different flavors, from the Umami Information Center. Guess which ones they liked?