I know Samba4 is listed as alpha but ... its the 15th alpha <g>. Its still scheduled to be released in 2011 according to the project timeline.
Watch the Samba4 Video's ... it only takes about 15 minutes but will easily demo how a Linux Samba4 server can become the PDC for a Windows Domain.
as I'm typing this the Samba.org wiki is offline but the URL to the SAMBA4 Video's is:
Joining Windows 7 to a Samba domain
I've found that there are quite a few organizations using it in production with good success.
Along that line I found out about an open source Resara.org
that has taken Samba4 and done quite a bit of integration & menuing work to provide a nice Samba4 Admin Console.
You can download the source but they also other choices including:
pre-packages VMs (vmware or virtualbox)
an Ubuntu PPA for Resara and Ubuntu 10.04 LTS.
Installing is simple and so is configuration.
You might want to give that a try and see if it will work for you.