I agree with much of what SapperCS already mentioned, and I want to add that you might be interested in joining Greatschools Gifted Group at community.greatschools.net/groups/11537
While there can be great variation in the ability levels of incoming kindergarteners, I do think you should find out about whether your school even over differentiated instruction (ie, a gifted/talented program) in later grades. If not, consider buying books like Susan Winebrenner's "Teaching Gifted Kids in the Regular Classroom" and sharing it with your child's teachers. (That book may be to address the needs of slightly older students, but I think there's a similar title about teaching Young Gifted Kids.....)

Make sure you put them in the best school in your area that you can afford with small class sizes, like minded students, and attentive teachers. The problem your child faces is boredom. As the other children are trying to learn the alphabet, your child will be under challenged.

Talk with your child's teacher and explain the situation. If you child is outgoing, perhaps they can be an aid to the teacher and help to keep your child interested while in class.

As well, it is on you to work with your child as much as you can to continue their learning, don't loose that gift your child has and let it stagnate, continue to challenge them.

Also watch out for alienation of your child, and ask the teacher to watch out for it as well. Your child will be different, and kids that are different become targets to the rest of the kids in the class.

Kindergarten is a fun time for kids, explain to your child that they should try and have as much fun as they can every day. Your child will find their niche and will more then likely do fine.

One last thing, be as active in your child's class as you can. Just your presence in the school, or the classroom will help bond you and your child and the classmates so much in those developing years, you will be greateful you did later on.