If I have a new system the only things I install are a browser, IM clients and some development environment. But the thing that has really changed now is that I can do the same thing on Linux without thinking too much. I can install Chrome/Firefox, the IM's are pre-installed and most of the development environments are available on Linux.
You could do that earlier as well but now the main thing that has changed is that there are new tools that are only available on Linux or Mac. The main softwares coming to mind are GIT and node.js. They are available on Windows but often seem crippled. If you really want to use them you will move to Linux or Mac. That is where things have really shifted. There are half a dozen twitter clients for Mac and quite a few for Linux but have a look around the only twitter clients you will find on Windows are Adobe Air based clients. No one seems to be writing native or .net based clients. To be fair there are some obscure .net based clients which no one uses but till date there is no official native/.net client from Twitter.
Quite a bit of the developer community moved towards Mac specially after the amazingly successful iOS. The rest moved towards web development. And as far as web development is concerned most of the newer projects are hosted on GitHub where being a Windows user really hurts.
The tide is moving towards Unix based systems with most of the hot new developments happening there. Its frankly been ages since I last installed a new software on Windows. Have not heard of any Tweetie or Coda for Windows. There is hardly any new software that has been specially targeted for Windows. There are half a dozen new apps that are released for Mac but don't think have heard a lot about any new Windows apps.
The other reason for the drastic shift has been the mainstream blogs. Most of the bloggers moved to Mac and the only thing they see is Apple and yet more apples. They cover technology as long as it is the only thing around. Check out Techmeme anytime of the day and all you will hear is Apple this and Apple that. Once in a while you might see a review of an app on Mac but there is hardly any news of any new software for windows. You might see a lot of reviews of web applications but then for those you don't really need Windows. You could be on Linux and they would just work fine.
Its just a matter of time when people realize that they can no longer use the newest shiniest piece of software on Windows that they currently use. In this case I am referring to IE9. Once people on Windows XP move to another HTML5 compatible browser and they get used to Chrome or Firefox there is little else that would hold them back on Windows. Ubuntu would run Chrome or Firefox just fine. With most app moving to the web I really don't see why you would pay for a licence of Windows.