I have installed the latest version of Zimbra (open source)
I have managed to get the GAL all shared and working.
Zimbra users can see Exchange user's schedules.
Zimbra and Exchange users can send each other meeting requests
My only hitch: Exchange users cannot see Zimbra user's Free/Busy
Here's what the log spits out:
[email@example.com;mid=4;ip=192.168.3.91;ua= ZimbraWebClient - IE7 (Win)/5.0.18_GA_3011.UBUNTU8;] soap - NoOpRequest
2009-08-25 15:02:44,893 INFO [btpool0-0] [firstname.lastname@example.org;mid=4;ip=192.168.3.91;ua= ZimbraWebClient - IE7 (Win);] soap - PushFreeBusyRequest
2009-08-25 15:02:45,272 INFO [EXCHANGE Free/Busy Sync Queue]  HttpMethodDirector - Redirect requested but followRedirects is disabled
So it looks like the settings are correct for it to connect (I pasted the generated path directly into a browser window and I got into the public folder area) but...
If anyone has any ideas, I would greatly appreciate it. I'm exhausted researching this "redirect" message.
Also, I have no access to the Exchange server and they are not cooperative. (hosted)
Global Interop settings URL: https://owa.mydomain.exchange.ms
O and OU: /o=mydomain/ou=First Administrative Group