It was a boisterous claim, but there is a movement. Two things I think are causing it...
1) I think though something major happened with the move to intel chips and a nix backend to OSX. Dropping PowerPC and the old Mac OS is something Microsoft couldn't do and now Windows is stuck.
2) Also major market changes that push it that direction.
- iPhone/Touch device and market
- Dynamic languages like python + ruby and rails/django web frameworks associated with them
- XBOX 360 caused people who were windows PC gamers to go console, making the main reason to buy a PC in the past (games) largely moot.
- New multi-platform game engines like Unity3D and now that Mac is Intel you can develop for both easily. (unity3d is what initially made me make the switch)
- The cloud computing paradigm (yes it is buzz but real) is pretty much not windows based, it pushes cheap nix OSes and a nix based OS as your base is much better.
- Distributed source control such as git that weren't and aren't very Windows friendly
- Finally good systems, right click, and killer machines (one dude I know has a 30 inch Mac screen and a 32GB 8 processor Pro, it is a monster). These things look like giant iPods now rather than colored beads.
- OpenGL is resurging with Khronos support (ES for mobile and all associated libraries)
Every developer that I know that has gone that direction has not come back, typically these are talented developers as well and long time windows people (myself included, C++ MFC, MCSD C++, MCSD.NET, .net developer for 8 years, did one of the first public .net sites with Microsoft for a large CPG packaging goods company, game developer on windows)
There are more reasons but I see the change, do you? The fact that you can run lots of great tools even Windows within parallels or bootcamp also added to the move.
Once you go Mac you never go...
Windows will still be needed for development for some time, but all new, fun and multi-platform development it is not required anymore. That being said I have two monster machines, Vista 64bit PC and an iMac and I need and use them both. I can tell you which one is more fun to work on though...