IF I send my daughter to preschool, she will go at 4. I think it is ridiculous that we are sending our children to school at such a young age. They need free play, that is how they discover and learn and maintain their curiosity. Learning needs to be self-directed, a natural result of interest and curiosity, and most institutional settings just cannot provide the necessary requirements to make this the case. In fact, I think many schools do a good job of killing any love of learning a child might have had.
I know a lot of my friends are sending their kids for the social aspects, but I don't buy the argument. With playgroups, and other available classes, tots can see other kids every day, if mom so wishes. My daughter sees other kids every day, and I use the money I save by her not going to preschool to do music, swim, and to buy memberships to museums.
During graduate school, I took a class on language acquisition. The kids that followed mom around (no matter the mom's socioeconomic status) learned more. You just cannot beat one-on-one dialoging. It is the most important factor for learning. In fact, study after study shows that class size is the most important factor in outcomes. Some explain this as being the result of the teacher's better ability to match stimulus (instruction) to interest. The only schools worth anything at the younger ages follow a child-center approach. This means that there would be very little "circle" time and the like, because the children are going to be at a very different places. Some of the kids in the room, for instance, already know their letters and what to learn phonics instead, etc.
I personally think people are sending their 2 year olds to school (who would have thought!?!?!?!?!?!?!) because (1) they want ME time, and (2) peer pressure. "Oh my god, so and so's kid is going to be ahead of mine!"
Like I said, if my daughter goes, she will be 4. I think we may just skip the whole thing and go to the Zoo instead. I know I am the best teacher my daughter can have at this point in her life, and there is no way I am entrusting the most important time period in her schooling to others. If I could send her to the Lab School (John Dewey's school in Chicago), or some school with instructors whose education match my own, I might think about sending her, but really, at the end of the day, I am the best teacher she can have, and like I said, one-on-one dialoging is the best educational device for young children.