Smaller FX shops that don't have IT staff are much more likely to consider it, unless they rely on software that isn't available in OSX, like XSI.
Which reminds me, now Houdini is available for OSX (it might still be beta, but it's coming if not here already).
The posers (fresh out of school, concerned about image/style still) usually are using Macs.
/ducks
Every design team I've worked with, startup or Fortune 500, has been on Macs. Yes, that's just more anecdotal "evidence" but I've been a consultant in several companies over the last ten years.
One Fortune 100 company I worked at had slowly moved the entire web development team (front end and back end) onto Macs. The front end guys liked Photoshop on the Mac, and the OS level colorsync support, and the higher gamut LCDs (versus your run of the mill Dells - no one was springing for the $5k+ displays). The back end guys liked that they could run the same bash scripts and ant execs on their local machines as would be running on the Solaris production servers. Having a 99% compatible bash shell available out of the box, and having the same path separators makes a big difference.
Plus, the corporate Dells + support plan + etc... are pretty close in price. And our department was responsible for 2 billion+ in corp revenue, so a few Macs are a small price to pay.
I'm glad you weren't supervising our department:)
The first reason was that they ended up costing twice as much to match the configurations (extra memory, the requisite software, etc).
The second and more critical reason was support. Apple just wasn't up to the task of handling a call from someone with somewhere between half a dozen and 100 macs at once.
The first reason has been changing in Apple's favor since, and the last time I checked around 4 months ago the cost was pretty much even.
The second I don't know, but one can hope.
And it would be a poor manager who didn't take advice from the specialists he employes in making decisions.