I am interested in how this is going.. I'm sure there are plenty of departments out there in a similar situation.
I'm looking for better integration with the rest of the company (ad/exchange) for my mac/linux clients. Getting these platforms to work directly with exchange doesn't work out all that well.
Having a zimbra box become a sort of 'man in the middle' between non-windows clients and the exchange system could be an exciting venture. Imagine installing some 'edge' zimbra servers that work with your companies exchange system.. if things worked well, they could end up replacing exchange completely!
If anybody has more information (thanks Anthony for your 2 points) that they could share, especially if you are or have implemented this, then holler away
Dallas