I noticed that Joel Spolsky repeatedly estimated the worldwide number of developers at 4 million in the Stackoverflow podcast. So, how many developers do the different platforms, such as Java, .NET/Windows, Flash, PHP, Python, Ruby etc., really have? This article puts Windows developers at 6 million, comment no. 2 in this article claims 2 million Flash developers and 6 million Java developers, and the second comment here claimed 4.5 million Java developers in 2005 and 3 million in 2003.
Now you can debate who's counted as a developer, and there are developers working on multiple platforms, but the number seems to be higher than 4 million by quite a bit.