Yep, the New York Times Personal Health section seems to advocate maximum dirt intake for the improved health of your growing baby.  Fair use except:

In studies of what is called the hygiene hypothesis, researchers are concluding that organisms like the millions of bacteria, viruses and especially worms that enter the body along with “dirt” spur the development of a healthy immune system. Several continuing studies suggest that worms may help to redirect an immune system that has gone awry and resulted in autoimmun disorders, allergies and asthma.  These studies, along with epidemiological observations, seem to explain why immune system disorders like multiple sclerosis, Type 1 diabetes, inflammatory bowel disease, asthma and allergies have risen significantly in the United States and other developed countries.

OK, first issue here is to reiterate the importance of the cum hoc ergo propter hoc fallacy.  Absent a controlled longitudinal study, all we have shown is, at best, a correlation that may or may not be spurious.  It does sound like there are some fruitful paths of future research here, given success in mice in utilizing worms to mitigate or reverse the effects of autoimmune disorders.  But don’t jump the gun and phrase the article such that you’re endorsing spoonfed bowls of dirt to Junior, pronto.

So what’s the evolutionary purpose of our infantile appetite for dirt?

“What a child is doing when he puts things in his mouth is allowing his immune responseto explore his environment,” Mary Ruebush… “Not only does this allow for ‘practice’ of immune responses, which will be necessary for protection, but it also plays a critical role in teaching the immature immune response what is best ignored.”

Ok.

“Children should be allowed to go barefoot in the dirt, play in the dirt, and not have to wash their hands when they come in to eat,” [Dr. Weinstock] said. He and Dr. Elliott pointed out that children who grow up on farms and are frequently exposed to worms and other organisms from farm animals are much less likely to develop allergies and autoimmune diseases.  Also helpful, he said, is to “let kids have two dogs and a cat,” which will expose them to intestinal worms that can promote a healthy immune system.

I’ll take a crack at positing an alternate evolutionary hypothesis: by testing our immune systems early and often, we help weed out the weakest genetic material by ensuring swift death of those with poor immune systems before they reach the unfortunate life stage of reproduction.  Of course, for many Malthusian-types, this explanation is a feature, rather than a bug.