While we'd love the whole world to instantly convert to Zimbra, we realize that from time to time people have that one department or peer organization running a different system.
To solve that it's time to announce our Free-Busy Interop is now out of beta - we're hereby officially supporting two-way free/busy information, schedules, and global address book access with Microsoft's Exchange Server, IBM's Lotus Domino Messaging Server, Meeting Maker, and a slew of other third-parties that interact with our free-busy API framework.
The query and propagation extension of free/busy data is done via REST and WebDAV interfaces. If you need more info on how to set it up checkout SRC/ZimbraServer/docs/freebusy-interop.txt (you can grab a copy via perforce / this post) or ask us in the ZCS forum section.
Kinda related: Don't forget that Zimbra side you can now visit http:// server.domain.com/home/username?fmt=freebusy to display an aggregate HTML calendar of the user's free-busy data. (Of course you can always choose to select "exclude this calendar when reporting free/busy times" if you wish.)