This has been true for a while if you count number of servers, but it's now true based on dollars: According to news.com, "computer makers sold $17.7 billion worth of Windows servers worldwide in 2005 compared with $17.5 billion in Unix servers."
It's worth noting, too, that 6.8 million of the 7 million servers that shipped in 2005 cost less than $25,000. That's probably not a surprise to all the SMB consultants out there who know how much a decent server costs! Microsoft server solutions have never been more affordable.
The full text of the article is at http://news.com.com/Windows+bumps+Unix+as+top+server+OS/2100-1016_3-6041804.html?tag=nefd.top.