This has come up here and with support, and I wanted to drop a line and let you know what the extent of the Support of Apple's new Leopard (10.5) is.
Zimbra Server is not supported on the 10.5 platform, and users who upgrade will experience trouble. Macintosh is a very important platform for us, and if you'd like to see a version of ZCS for 10.5, we encourage you to vote for Bug 21441 - Requesting Apple-Server (10.5) support for ZCS , and cc yourself on it.
The iSync connector for ZCS 4.5.9 supports Leopard, so that shouldn't be an issue.
Also, 5.0 will have support for Safari 3 with the Advnaced/AJAX client.
We hope that users who have issues will post in the forums so that we can try to help you out.