Samba v.3 is able to be a NT4 style domain controller. If you had a AD server running for Exchange, that is not good enough.
Samba v.4 will be able to be a Windows 2003 style domain controller, but is not done yet. Not by far.
Next question would be: do you have any Windows clients left? If so, you have a problem. Windows is not as pluggable as Linux. While it is possible to change a certain dll file (I forgot the name) to authenticate against a generic KDC, Windows was built to work with AD and with AD alone. Anything else requires altering Windows system dll's. That sucks.
If you do not have any Windows clients left, it becomes a lot easier. You can easily replace Windows AD with a combined Kerberos / LDAP solution. Kerberos kdc (Key Distribution Center) packages are in all distro's. LDAP servers are available in a lot of different forms. OpenLDAP server is in most distro's. A GUI based management tool for you LDAP directory is available from a lot of open source LDAP serers, like 389 and I think Apache DS too.
I mentioned the FreeIPA project in this context in another thread as an integrated solution, but it is only for Linux.
So, to make a long story short: do you have Windows clients on your network still?
Edit: Apparently not. So, build yourself a KDC, grab a copy of 389 DS and you're good to go. Then, you'll have to do some LDAP scripting to pull user information from the domain controller and insert it into your LDAP server. I don't think you can migrate the users' passwords though, you will probably have to reset those.