IPA v3 will bring trusts with existing AD setup, that's first step. It will not be full blown AD DC but AD will treat IPA v3 users and machines as if they are from a native AD forest which will be enough for majority of use cases.
Getting Samba 4 AD DC out as proper AD DC is also important task. However, it should be viewed also in a perspective of being an application in a larger setup -- if you would run it in an isolated VM, you can already build your own samba4 package in Fedora for that purpose by changing few arguments to build process. It will use embedded Heimdal kerberos implementation. Unfortunately, you then cannot share the same VM with anything else relying on Kerberos in Fedora as those will be build against MIT Kerberos and use by default features unavailable in Heimdal like DIR: credential cache collections.
So in isolated environment it is already possible to build and use Samba 4 AD DC in Fedora 18 (Rawhide right now, of course). Integrating into distribution is a bit large scope and requires more effort.