At the moment no, there aren't any plans to drop dependency on Postfix for the Linux side.
There are dozens of
add-ons, documentation manuals, how to guides, and third-party policy servers out there designed to work with Postfix's API's. We wouldn't want Zimbra admins who rely on those items to suddenly be left out in the cold / be forced to modify all the software to interact with the new MTA.
Plus getting to that point has taken their open source community a significant amount of time. While saleability testing of the new mail-transfer-agent will be QA's primary focus in order to bring it out of beta for the Network Edition, we'd still rather see a 'Postfix for windows' implementation that we could revert to instead, and thereby focus our engineering efforts on other aspects.
Introducing ZCS for Windows Server » Zimbra :: Blog