Weblance
Ask yourself "Why do I want Active Directory?".
It is a Microsoft management tool and a very good one, too - if you are running a Windows Server. It provides the functionality needed to manage large (huge!) domains.
SME and Samba provide a full Domain Controller functionality (as it used to be in the old Windows NT days). This is more than adequate for managing small domains - and 10 workstations is a small domain. Security policies can be set on the Domain Controller (in this case SME).
From your original description, I think you have simple system requirements - fileserver, mailserver, webserver. SME does that beautifully.
You also need to handle logins and control access - it is all there in SME. I use an addon (contrib) called Smeserver-tw-logonscript (look in the Wiki/Contribs page). I am not the developer - just a user. The addon allows you to map Windows drive letters to SME Samba shares -depending on the groups the user belongs to.
Here is the original announcement of the package from the forums:
http://tinyurl.com/37pck9hRemember - your problem is really "Can I replace a Windows 2003 server with an SME server and still have an easily managed network?" NOT "Can SME server run AD?"
And the answer is YES.
Another suggestion: set up an AFFA server for backup. (
http://wiki.contribs.org/Affa)
This is a brilliant piece of work - it is an addon for SMEserver which does scheduled backups (and allows retrieval of deleteted items still in the backup cycle) AND is a lifesaver if the main system goes down. An AFFA server can be "risen" to become a fully functional clone of the main server in minutes - including all user data. I have used this a couple of times and the longest it took for 500Gb+ was less than half an hour. Try that with a Windows server!
Cheers
Ian