My colleague, David Morgenstern, just posted Apple in the enterprise: Living without Xserve and I found myself compelled to add a comment or two.
Apple has often found itself at odds with large enterprise IT departments because the company really doesn't understand how they work or what they really do. So, they've bumbled about, occasionally won some business, and then proceeded to do things that made their customers look bad. The unsung heroes of IT have long memories and hate working with suppliers that make them look bad. Apple seems bound and determined to make the IT folks who made an Apple server decision look bad.
IT staff are tasked with keeping the lights on and making sure that everything runs smoothly in spite of change coming at them from every quarter. Obviously, doing this successfully takes careful planning and careful execution.
The IT folks expect suppliers to share (at least roughly) their plans and expect them to act in a consistent manner. Apple doesn't offer much insight into what it is going to do and makes surprising moves all of the time making planning problematic. So, the decision-makers in IT routinely select other suppliers.
It is clear that Apple has forced its way into the enterprise at the smartphone, the tablet, the laptop and, to some extent, the desktop PC world. End users like the experience and are bringing the devices into the enterprise regardless of the views and concerns of the IT organization.
It is also clear that Apple's approach to server-based computing, with virtualized environments, openness and a few other areas are self-imposed obstacles in Apple's membership in the enterprise computing suppliers club.