At UDS, there was talk (mostly from Dan Shearer) about using Openchange to provide a mapi proxy to MS Exchange. I find this interesting. If Dovecot/Postfix could use it, it would allow *NIX clients to talk to Exchange without turning on OWA. This would delight sys admins on both sides of the fence. Of course, Outlook clients could also talk straight mapi to the proxy as well.
I like the idea of abstracting MS Exchange, because once we have all the clients talking to us instead of MS Exchange, it makes it much easier to replace exchange.
The Openchange folks have made an announcement.