We are an NE customer having chosen Zimbra over other solutions. It is far too early to panic and make rush decisions and declarations, however I would like to answer the post saying that no one should worry because Zimbra is a "boutique" company/product only recently acquired by Yahoo!.
Put yourself in MS's shoes, and look at their past tendencies of copy/acquisition. MS is loosing existing and potential customers from Exchange to Zimbra because it is a product which offers a better solution with more options and choices to the customer. Would it not be in their best interest to stifle competition by making moves or choices to affect the viability of Zimbra in favor of the product they have invested so much into (Exchange/Outlook)?
We have all seen the reaction to ODF, a nice format guaranteeing the availability and accessibility of data through the future, giving the choice to the customer. This threatens the "monopoly by lack of choice" and the "lock in" by incompatibility. These are two of the primary things which helps MS stay profitable. Outlook is one of the few strongholds which keeps MS's foot in the door. With the popularity of Zimbra growing and OOo v3 coming around the corner with "Outlook functionality", it is vital for MS to keep customers tethered through any means possible.
Whether this is going to happen or not is of course unknown but just as it is too early to claim that it is what is happening, I would say it is far too early to say that it isn't a possibility or end goal.
In either case Zimbra and their customers are along for the ride. Let us all hope for a good one.