We live in Warren (near to you) and my son attends preschool sponsored by Warren public schools. He's already learned to write his name, recognize the letters of his name out of order and put them in order, basic spelling, some basic reading skills, writing letters (upper and lower case) and numbers, recognizing and drawing shapes (circle/squares/triangle/etc). He's always been pretty advanced in language skills (using words like non-negotiable, atrocious, acceptable, etc) since he was mid three yrs old.
My son is a February birthday, but kindergarten here must be 5 by December, so he'll be an "old" kindergartner too (love that term). I actually think the Jan/Feb/March babies get a little more advanced education from their additional time in preschool, but at the same time they DO get bored because they re-learn a lot of the same stuff over again. Therefore, I work with him at home on telling time, reading, etc. to keep his little mind busy. In preschool, they seem to teach as advanced as the slowest students, and since I know some of the kids who've never been to a "school" setting are a bit behind, I know he won't learn that there.
I actually tried to call Warren schools to see if he could test in to kindergarten early in September 2009 (I KNEW he was ready) and was told the State of MI doesn't allow early starts. So we're stuck patiently waiting for September 2010 kindergarten.
If you can afford it, I think the idea of an advanced preschool or Montessori (as suggested by another mom) are great ideas. We toured a Montessori school when I was still working (and could afford it) and those little ones are AMAZING!!! The kids demonstrated their skills for us and were SO ahead of the game. Most were READING at three and four.
Basically, your right. Preschool is mostly focused on social skills and getting used to a school environment (following directions, sitting patiently, etc). On the upside, I was told by my son's preschool teacher that some of our "really advance" older preschoolers are great at assisting the others in their education and will actually help the slower ones strive to improve. So its beneficial to the preschools to have our smart ones there LOL. Too bad they can't earn a paycheck, right!