At my location all the COMPANY graphics computers are PC's, but a few Mac Fans bring in their OWN Macs to use at work.
I'm not bashing Macs, just pointing out that especially since the economic downturn, companies are a lot more interest in lowest cost which PC has the edge on for 2 reasons. PC's are cheaper to start with. And PC makers are willing to offer discounts which is something Apple will not do.
But full disclosure, we no longer have graphic artists anymore. We used to have 4 at our site. We create most of our own graphics on PCs using internet based software. More complicated orders are done by graphic artists in our central graphics location which serves 42 TV stations. We also have 82 daily newspapers and 443 non-daily publications. Throw in that each of the 42 TV stations has a main, and at least 10 sub-websites, and each of the 82 papers runs a website, we do a whole lot of graphics, all on PC.