Since Sogo now has native Outlook support, without a connector, through the integration of OpenChange, maybe the time is right to look at possible integration with Zimbra.
Now, an obvious problem immediately comes to mind, which is that Zimbra sells the Network Edition which includes their own proprietary connector. This is fine for now, since OpenChange does not seem as feature rich or mature as the current solution, but it would seem that future inclusion of OpenChange in the core product would be the right way forward.
Because Outlook support is one of the last missing features of the community edition, I can see one of two things happening: Zimbra start creating a community edition with things like Z-Push and OpenChange integrated (probably not), or a community branch is created which includes these things. The other main features of the NE (backup and archive & discovery) do not yet seem to have an equal in the open source space.
Personally, I have no problem with paying for a product that is completely open source, especially with the great support Zimbra offers. I also think the transition to complete open source is starting to look inevitable, since most if not all components now have open replacements, even though these are not yet equivalent in functionality.