Google Search that led someone to my blog:
Can a student be released from [local school district X] and go to a school of choice?
I don't think I've fully answered this question, so here is the short answer: You don't need to be "released," so to speak--there is no requirement that you go to public school in your school district. You could choose a private/parochial school, or a school of choice in a different school district--the requirement is that your child gets an education (and that includes the possibility of home schooling).
Depending on the school district you would like to opt into, you follow their enrollment/application guidelines. They can generally be found on the school district's web site. For instance, Lincoln Schools and Ypsilanti Schools have "schools of choice" buttons on their web site. (Although Ypsilanti's form and information is outdated, I think the procedure hasn't changed). I believe that they will notify your "last school" that you now have a new school. One other thing--typically, if your child has been suspended or expelled in the last two years it will not be so easy to switch them.
Note added 8/6/09: Here are the details from the state's web site.