From 1997-2009, I called on about 100 customer sites.
From this small, but faily representative, sample, the percentage of corporate desktops does not appear to have changed much:
1997: Windows 99% Mac 1%
2009 Windows 99% Mac 1%
The only people using Macs are designers, engineers, and a few programmers. They insist on them for their specialties. Everyone else gets a PC with Windows.
In the corporate world, MS doesn't have anything to worry about (yet).
I think this where the compensating trends of students preferring laptops to desktops is coming into play. OS X has a better laptop experience. I hate waiting for Windows to come out of hibernation. I hate waiting for Linux drivers to appear and work. It's nice having the reliable drop off service at the Apple store, since laptops break more frequently. Apple simply doesn't sell "normal" desktop computers...
A problem that is obvious and growing should be "worried about" immediately. If you wait till the problem is gone and you've lost, it's too late.
At my last job (at a company of 500), I used MS for office tools, OSes, and server side services like mail.
At my startup, as the "IT" guy, I live on google for free and Apple for cheap. I curse under my breath when I get a Word Doc or PowerPoint instead of a PDF. My company can scale to 500 on these practices, which means Microsoft has a great deal to worry about. Right now.
[edit: correction, I use Windows on a cheap PC to periodically test how broken IE is. Soon that will be a convenient web service]
I am pretty sure freshmen are required to have a laptop and the Mac bundle deals at the computer store are pretty irresistible.
According to http://www.fsucs.com/ the Apple bundles are cheaper than Dell or Sony which are the only other options.
So Apple is discounting for students, looking to snare them for life.
Also this is posted "Due to circumstances beyond our control, all Apple orders may experience a significant delay."
Can't make'em fast enough!