If you use Mac OS X 10.5 (or, presumably, later), then you can use the iCal interface to interact with Zimbra. This can be a nice feature in and of itself, but it also provides a way for Blackberry devices to syncronize with Zimbra, albeit indirectly.
To set up iCal to syncronize with Zimbra:
Select iCal-->Preferences, and then click on "Accounts."
Click on the "+" at the bottom left of the window to add a new account.
Enter a description (such as "Zimbra Calendar").
For a username, enter your Hampnet ID (mine, for instance, is "kmmLO")
Enter your Hampnet password (this is the password you use when logging onto WebMail or the Intranet, for instance).
Click the triangle to the left of "Server Options," and then enter "connect.hampshire.edu" for the Account URL. Do not check Kerberos authentication.
Click "Add." If you receive a message that you are connecting in an insecure manner, select "Connect Anyway." If you are prompted to store the information in your keychain select the option you prefer.
When you have been successfully connected to the server, you will have an option to set the frequency of synchronizing your calendars. Select the timeframe that best suits your work style.