I for one, welcome the new Yahoo overlords...haha j/k
Seriously, though, it's like Mike said, you usually won't end up seeing anything different for about a year. My guess is the first major noticeable difference will be with post 5.0 ZCS products, maybe 5.1 or 5.5(don't know how Zimbra plans to number after 5.0).
Some of the pro's I see in this are:
-Zimbra IM collaboration can be really great with the help of Yahoo Messenger
-Better work on large deployments(we have over 30,000 accounts so that's important for me)
-Better Spam filtering. I know it's great already, but Yahoo does a good job on their end too(my experience as a yahoo user)
-More Zimlets...what can I say, I like them
-more people to look at code and fix bugs
Some of the con's I see are:
-the current uncertainty in where things will go could make possible new customers nervous(I hope that is short lived though)
-Could turn into Yahoo's new mail interface which I am not a big fan of
-Yahoo not paying attention to the little guys out here...never forget about the little guy
For those worried about the conglomerate buying the startup, time will the best decider on that front. I suggest patience. But in the end, it is you that makes the decision where to go. Go where you are most comfortable. My experience in working on products that are bought by big companies(ahem...Cisco) has so far been good. I know Yahoo isn't the same, but I expect there to be a good ROI for them and Zimbra in this...with an end result of very happy customers.
I am the mail administrator at our EDU and have been very happy with the product, the support, and what I see coming in the future. I expect the high standard that has been set so far by Zimbra to be kept.
Congrats Zimbra and keep up the great work!