Zimbra 6.0 will be out before Snow Leopard, the only people able to answer this question before that is released will be developers who are seeded with the early releases and I dont know if anyone in Zimbra HQ will get that. If they do, then a straight answer can be given.
If to that point, if we take what we know, then I'd go with an answer of "yes, out of the box".
The iPhone currently believes that Zimbra 5.x is an Exchange server, and Zimbra 6 will make that even better as it seems to support the extra bits of the protocol such as remote wipe, shared calendars, GAL access etc. It's reasonable to assume that as the iPhone runs a version of OSX, then OSX will use the same machanism to access Exchange as iPhone does. Thus is iPhone works with Z6, then so will OSX 10.6 |