I'm a registered developer running the latest seed of 10.6 I got in san francisco last week. I'm also running the latest zimbra ne with a "real" signed ssl certificate, and I also have direct vpn access to the server.
The first three times I tried to use the exchange setup wizard on 10.6 in automatic mode it wouldn't connect to zimbra. The only Exchange server setting is for 2007. It continually fails to connect to server. There are settings for internal exchange and external exchange settings. I use my local IP through my VPN in the internal setting and my external ssl-signed mail server settings in the external setting.
Either way, it ain't workin'. Is Zimbra "Exchange 2003-like" or "Exchange 2007-like"? At only $29, a lot of people are going to be on 10.6 the day it comes out.