Stuart Herbert has been studying adoption rates for Windows use on PHP and, in response to comments made in this article he's shares some of his own stats on Windows adoption (at least in the European market).

[Remi] was expressing his surprise at how few people participated: "I refuse to believe there are only a couple of dozen PHP developers in Europe." It's an odd statement...the world and his dog knows that PHP is immensely popular over here in Europe. Perhaps Remi completely overlooked the Windows factor here?

Stuart points out that he's seen around fifteen percent of developers using Windows as a development platform, but only three percent use it as production server. There also seems to be a lack of interest in the whole PHP-on-Windows space.