In a word: support.

I went into this issue in more detail, and with earlier versions of Apple software, a few months ago. But the rash of blue screen (not a term traditionally associated with OS X... it's a dark day in Apple land when the least positive attributes of Windows start showing up in their beloved BSD-based OS) system lock-ups that have broken out as people attempt to upgrade to Leopard put me in mind of it again, and the fact that I hadn't addressed this most important issue in my post on Leopard's enterprise potential a few days ago, put me in mind of it again.

That previous post dealt specifically with SMBs but the issues affect enterprises as well, albeit to a lesser extent. Those issues are, in a nutshell, Apples horrendous response to these sorts of failures and the poorly structured support environment for both Apple hardware and software.

For example, read Dana Gardner's experiences with the upgrade.

PCs, and their various operating systems, whatever their drawbacks (or, in fact, probably exactly because of their drawbacks) have a well-developed eco-system of support, both offered by manufacturers and thousands of specialty businesses small and large. I can pitch a brick out my front door right this very second and cold-cock a halfway competent PC technician.

If I have an issue with my Mac which I can't self-resolve, however, I have Apple. And Apple aren't answering their calls just now. As far as third-party support, I suppose I could call the three or four local techs I know who are more than passingly familiar with OS X and Mac systems, but they'll be swamped already, and that leaves me shipping it off somewhere and without a laptop for a week.

This is real problem for corporate IT considering adopting Macs. Sure, the planning and testing are better in corporate IT environments and this particular problem probably hasn't come up. But if something does... well, as a counter-point, I had an HP server burn out its GPU this morning. Two hours later I had been in contact with HP support and had a commitment to replace the board or the server as required. If the client had a higher warranty level, it could have been taken care of next day. Apple simply doesn't have that sort of system available, even in the US, and can't offer that level of service even if they were so inclined... and it's not clear that they are.