I think one reason to keep a kid in kindergarten versus first is your child's ability to sit still in a classroom. Although our DS4 knows the entire kindergarten curriculum, we're still going to wait until he's 5 and send him to kindergarten. Part of his learning involves running around when he learns something new, and he would think we were sending him to jail if we told him he had to sit still for several hours.

Also, there's a lot of fun stuff in kindergarten too, and for the academic portions, we're hoping he can be accommodated while still enjoying the fun stuff. (Of course he's not eligible for kindy based on age until 2009, and we're also teaching him stuff he wants to learn at home, so our plans might change by then, but those are our current thoughts.)