shrugEvery 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.