For decades, the Alt+Tab order was the same as the Z-order, but that changes in Windows Vista if you use the enhanced Alt+Tab feature known as Flip, which is on by default on most systems. There are three types of interactive task switching in Windows Vista:

  • Classic Alt+Tab: This is the same one that's been around since Windows 95. It shows a grid of icons.
  • Flip (new for Windows Vista): This shows a grid of thumbnails.
  • Flip3D (also new for Windows Vista): This shows a stack of windows in 3D.

Classic Alt+Tab continues to show the icons in Z-order order, but the developer who wrote Flip told me that Flip changed it up a bit based on feedback from the design team. The first several icons are still shown in Z-order order, but if you have a lot of windows open, the rest of them are shown in alphabetical order to make it easier to pick the one you want from the list.

I think it's a good sign that nobody seems to have noticed. A lot of user interface work tries to be invisible.