Exchange 2007 with Mac OS X PDC and Open Directory

Hi, I hope somebody can be of some assistance. We are running a series of Mac OS X servers with Open Directory master and replicas and a PDC with multiple BDCs. Currently we are using an IMAP based mail system using Exim running on Ubuntu 8.04. For many reasons, we are wanting to migrate to Exchange 2007 for mail, but without introducing Active Directory. Is it possible to run Exchange 2007 and plug it into the LDAP tree on our Mac OS X Servers? Would we set up the Windows 2008 server running Exchange as a BDC? I would love to hear from anyone with experience!