We went through this same debate with my now DS6, and we're starting it again with DS28m. Here's my 2 cents.
I think the question is what, if anything, do you expect him to gain from his preschool experience? If you don't think he has anything more to learn at a 2 or 3 year old level, do you really need to send him to preschool at all? Would that time be more productive in playdates, co-ops, and other social enrichment classes?
We debated sending DS6 to 3 year old preschool when he turned 3 because we didn't think there was anything he'd gain (especially after watching him in action with his peers at a 2 year old mommy-and-me preschool). It turned out that sending him was one of the best things we ever did. His teacher had 20 years of experience, and she was a Godsend. She clearly recognized his gifts (pushed us for IQ testing, which is how we came around to YS), but also opened our eyes to "regular" 3 year old skills that he could continue to use (ie how to put his coat on by himself and zipper it!, how to play with others when they didn't want to (or couldn't) do the same complex play you wanted them to, etc). Since it was a Catholic school, he did learn some new things like prayers, plus sign language, etc. The greatest thing, though, is that he had a wonderful time, made good friends, and enjoyed some of the simple preschool pleasures that just comes from that age. When he wasn't in preschool, we spent hours every day engaged in field trips, enrichment classes (music, art, etc),and/or playdates with friends, or just had fun with projects and play around the house. I often placed him in higher level groups for challenging activities like nature and zoo camp and library class, but didn't want to get into the whole acceleration issue with school since I knew it wouldn't make any big difference whether he started a little early or had that extra time with me.
We took DS's 2nd semester of 4 year old preschool off when we moved. He was far beyond the regular 4 year old curriculum, and it didn't make sense to spend $300/month for it when he wouldn't have been happy w/ his peers by that point. Pulling him didn't hurt him one bit, since he placed at a 2nd-3rd grade level when he entered K a few months later.
DS28months is in a different situation. He's also likely HG/PG, so we're in the same decision-making process again. He is going to start a 2 1/2 hour preschool co-op one day a week (where I co-teach 1 of the hours), sit in on DS6's co-op 1ce a week where they are doing science, spanish, music, etc., and he has his own playgroup once a week. Since we usually do some kind of field trip once a week, too, I don't really think he needs a regular preschool program. However, I do like the idea of him getting some kind of classroom exposure, so I'm considering putting him in a 3-6 year old Montessori 2 mornings a week next year.
The bottom line with preschool is that children go for the fun and socialization (which any kid can always benefit from). If they are advanced, just about everything they are learning comes from "homeschooling" anyhow. At this age, it doesn't necessarily mean that you have to go out and buy workbooks, but they'll get much more from us with playdough, cutting/pasting/painting/writing at home as they would in just about any 2-4 year old classroom. If DS likes to go to school, I'd probably place him with his age peers for now and consider it a fun type of playgroup.
