In my experience, even if you go in ahead of time and let the teacher and principal know that your kid is already reading at a third grade level, they will still want to get to know your kid on their own. Depending on whether you have a kid who shows his stuff or who likes to blend in quietly, you could have very different outcomes. Personally, I would give examples ahead of time of what types of things your kiddo is working on/reading, and say something like, "I'm not sure if that's normal or what - will there be other kids like that too, and do you group kids based on ability?" Make sure you go in asking for advice and thinking of it as a partnership -- teachers will quickly get offended if you sound like you're just telling them how to do their jobs (even if you didn't intend it that way).
Since you're in an area with a lot of highly education folks, you may get lucky and have nothing to worry about. However, in most places, kindergarten is about letter of the week and very basic math. Kindergarten teachers are used to many different levels of kids in their classrooms, though. But usually not so familiar with the way-out-there kids.
