samba - Feb 2004 - Replacing an exchange server as the domain controller

I've recently started a new job and there is currently an exchange 
server acting as the windows domain controller.  The problem with this 
is that it wants to control everything from ldap to dns to email...  I 
want to migrate to an opensource (probably bsd) server where I will be 
able to get things back to a reasonable state with only one ldap and 
one dns...  Has anyone done this?  If so do you have any documentation 
on it?

Thanks
Mara