Holy cow, I wrote a book!
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 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.