Recently one of my mentors tweeted a link to an article entitled "Building and dismantling the Windows advantage." Lots of statistics and a good read. Don't worry. I'm not going to use big words like pl8t0 did.
I have a couple of thoughts on this.
First, the transition from Windows to Apple happens in "thunks." For example when I moved my wife from a BlackBerry (because Google dropped BlackBerry support) to an iPhone I should have heard the "thunk." The next thing you know I'd sold my hacked TouchPad running Android and bought an iPad. Now the iPhone and the iPad are connected (and I use that term loosely) with the iCloud. The "thunk" happened easily at my house. "Thunking" in an enterprise won't be so easy.
Current technologies encourage homogeneous platforms. Even Mac and Linux users who interact with other businesses tend to converge using Microsoft Office file formats. Remember back to the migration from .doc to .docx. (You have done that haven't you?) Today making an enterprise "thunk" from PCs to Macs is a big transition.
A side issue on this is that enterprises like to multi-source all their purchases. This doesn't necessarily mean that they have 2 vendors' products sitting side by side but that they have 2 vendors at least bid on a product, in this example a desktop PC. They may award all purchases to a single vendor for a fixed time (several years) then rebid the product. Since there's only a single source for Macs that would be a problem and require some rethinking of the acquisition process.
Windows 8 will be a mover and a shaker in this arena. It could go either way. It could frustrate enterprises with the forced Metro interface or, if Windows Phone is adapted by the enterprise, complement the mobile environment. Too early to tell. Why don't you get me a Microsoft Surface for Windows 8 Pro and I'll give you some feedback?
Second, I don't think the long term play is full function desktop devices. The "cloud" is the end game. I was speaking with an executive with a leading IT vendor recently and she was saying that she didn't understand why the iPad was taking off in the enterprise. She couldn't "do" anything with it. I pulled out my Android phone and showed her my instance of Epistle. On the iDevices I use Nebulous Notes Lite. And on the desktop/laptop I use Wordpad. These apps leverage Dropbox to give me seamless editing of simple text documents wherever I am on whatever device I have at hand. It's an incredible experience. I understand that she was looking to be able to edit PowerPoints and Excel documents but that's just an issue of maturity in these applications. With the "cloud" it doesn't matter what your client is.
I understand the enterprise concern over "cloud" security but I forsee that being overcome by private clouds, e.g. SharePoint.
Remember the Chinese curse, "May you live in interesting times." We certainly are.