Got a customer server coming in that's exchange and I want it to talk to the existing setup, users hate more than one login!
Can Exchange 2010 get authentication from OpenLDAP? (running on CentOS 5.5) The Exchange sessions will be virtualised and moved from it's current chassis. We are currently familiar with configuring Samba, kerberos, winbind etc for samba shares.
Exchange expects Active Directory. You're going to be fighting a major uphill battle to try to get it to work with a non-Active Directory LDAP server. It's certainly not a "supported" configuration by Microsoft.
(If you have any number of Windows clients you'll probably appreciate having Active Directory just for Group Policy anyway. Nothing against samba and the FOSS solutions, but they haven't gotten full Active Directory "emulation" working yet and, frankly, the functionality that Active Directory provides for managing Windows clients is a big time saver.)
Evan's answer about exchange expecting active directory is exactly right. Aside from support I think there are a number of commands and schema attributes that are not implemented the same or at all. I tried getting exchange 2007 working aganst openldap, after a few weeks I gave up.