Fedora Core 389 is close to your needs, check it out at: http://directory.fedoraproject.org/

Like you I'd like to create a real AD DC, OpenLDAP is powerful if you compare features and options you've but if you'd like to replace a real Microsoft Windows Server controller it's a pain in the ass.
I don't mean just single sign on authentication but also group policies and FSMO roles, one of the big advantages of an Active Directory struture is the Group Policies unified management and control. I've tried to do some tests with LDAP and Fedora customizations but I'm quite far from a genuine AD server.

If you want to test something quick go with the Fedora 389 project, if you'd like to be more distribution independent OpenLDAP is the way to go.... but we're still far from Microsoft side

Forgot to mention another thing:
I work with AD as many of you, I'm a sysadmin and I've Windows Server machines to manage even if I'm a Linux geek. I'm trying to create some sort of working group for building a project based on OpenLDAP with patches and tricks to create an empty AD tree, then I'd like to use this tree and Microsoft Management Console tools to handle it, anyone interested ?