You can use the System.Web.Security.ActiveDirectoryMembershipProvider class to do Membership against active directory!!!
However, you have to setup a special connection string to point to a domain controller and to run as a user to query LDAP.
I’ll be working with this a little bit later and will likely post the code for this project somewhere…
Here is more info on how to do this:
http://msdn2.microsoft.com/en-us/library/system.web.security.activedirectorymembershipprovider.aspx