Our older son went to "playschool" when he was three and part of four, and we've considered it for our younger son, who is turning four. We aren't terribly concerned about the "social" aspect of school, as our kids have no problems interacting with people (adults or kids their own age), and we are a part of a homeschool co-op.

We know several other homeschoolers that have done "normal" preschool for their kids, too. I don't think it's an either/or situation--in fact, it may give you some insight on how your kids learn best (we discovered that our son does really well when taught something one-on-one, but can often become frustrated in larger groups).