I watched and enjoyed the keynote speech and figured that if it supports IMAP, then it'll work just fine with Zimbra email, no mention of how it read calendar data was given though.
I dont want to speculate on a device that I cant buy yet, but I'd really like it if Zimbra was fully compatible with this thing - I think it's reasonably obvious that the device will be popular. We have people in work wanting IPaq's so they can see their email and calendar's (I got the email working easy, I gave up on the calendar) so if Zimbra make the iPhone work fully (in the OSS version) then I personally would be a very happy guy
