I haven't seen any recent stats for the markets in general. I have however noticed a shift at my work, the Mac admin at an international non-profit organization with roughly 500 end users.
3 - 4 years ago the percentage of Macs would have been well under 10%, likely in the 4-5% range. About 3 years ago we started offering Apple options when people were due for a new computer (typically 3 year cycles). As of today Macs account for about 40% of the computers in our organization. The majority of new computers seem to be Macs to replace PCs, mostly because we're just now starting to see Macs replacing Macs that have aged past the 3 year cycle.
With that said, we're still heavily entrenched in Active Directory and Windows file shares with only minor management performed from our primary and secondary Mac servers. I don't see this changing any time soon, especially since I doubt Macs will move much past the 50% mark. Maybe 7-8 years down the road as more entrenched people retire and new faces join.