To add a little 'spice' to this thread:
we've recently chosen Zimbra as our new platform for messaging, coming from Lotus Domino. Now in the middle of migrating our users from several domains and ... testing zimbra mobile against various devices and clients as alternatives for our Domino-Blackberry integration via BES4Domino-server. As it is: coming from BES, or better: being on BES still, especially the various clients are 'not quite' up to comparison with native BES (not very new info I suppose..).
A little more info on the integration-module would be (widely) appreciated on we also would be keen on participating in a beta-program if you're looking for volunteers..

Seriously: going to Notify adds a lot to the TCO of the messaging solution (relatively) and next to cool it'd be an interesting cost-saver for companies if a native zimbra-solution would be available, moreover since communication between server and device (always (!) via RIM as noted earlier in this thread) adds to the cost of the solution per device/user (which we consider appropriate in our opinion for the extra value the blackberry still offers)
Anyway.. Looking forward to this extension a lot!!
Cheers,
Peter