So for most white-collar workers working in companies whose IT staff is able to deal with sourcing and supporting two platforms and handling the fairly minimal integration between Mac OS and Windows, the Mac can be brought in as-is.
But the more controlling and/or farflung an organization is, the harder it is to support Macs. Standard asset management tools don't have good Mac clients, for example. Remote management of a Mac beyond what an Exchange policy can enforce also becomes tricky, due to the lack of good Mac client management support in Windows Server or in tools such as HP OpenView, IBM Tivoli, LANdesk, and CA's management suites.
The departmental focus of Mac OS X Server also doesn't lend itself to central management across multiple office locations, and the need to bring in a Mac to an Apple Store or authorized repair shop also becomes problematic outside of major cities or when there is no local IT staff.
And the Mac's lack of presence outside basic applications also becomes an issue. (Apple maintains a list of business Mac apps that's worth perusing to see if your apps have Mac versions.) Although some specialty apps, such as IBM SPSS's analytics software, have Mac versions, most don't -- or they have only limited-functionality versions. Office 2008 for Mac is a great example: Not only does it not support Visual Basic, but its Exchange client, Entourage, doesn't support away notices or allow users to see which addresses are contained in an Exchange-hosted group address. These partially capable clients make it impossible to assure everyone has the same capabilities, and thus creates exceptions that IT has to manage.
Thus, the more applications your organization uses, the more of a headache these Mac software issues create. That's why in large companies, the Macs tend to be clustered in specific departments, such as marketing, where they can be managed locally and for which the specialty software the users need is available for Mac OS.
iPhones faces the same scale ceiling
The situation for the iPhone is no different: If remote management, compliance adherence, and large-scale management are necessary, you're out of luck. There are no native Lotus Notes or Novell GroupWise clients for the iPhone, just limited-capability Webmail access. The iPhone's Exchange policy support is much better than what Google Android or Palm WebOS provide, but nowhere near the level of BES (BlackBerry Enterprise Server) or what Windows Server can do with Windows Mobile.
You can distribute configuration profiles to iPhones via email or Web sites that contain Exchange ActiveSync policies and VPN settings, but you can't monitor whether users have installed them or track what version they have -- something many public companies must do to meet various compliance requirements. Having a local IT person hook up each iPhone to a USB connector and view its settings in Apple's iPhone Configuration Utility is not a workable option in large businesses, where their scale requires automated management.