Nope, that didn’t fix it.
I did realize, while messing around with it, however, that my initial post wasn’t completely accurate. As far as I can tell, it’s not actually alt-tabbing that is causing this behavior, but rather the “minimize” button in the upper right corner of the ZBrush interface. It seems that I can alt-tab to other programs no problems–even to the desktop (which minimizes ZBrush automatically)–and then return to a fullscreen ZBrush. If I minimize ZBrush with that button, though, there seems to be no way to return to fullscreen short of quitting out and restarting. So I guess the workaround is simply to make sure to never use the minimize button in the ZBrush UI when I want to switch to other programs.
I’m not sure of this button is bugged just for me, or for everyone. Here’s an example of what I am talking about:
ZBrush when launched:
http://www.odditorium.net/zbrushwindow01.jpg
ZBrush after using the minimize button and then maximizing again:
http://www.odditorium.net/zbrushwindow02.jpg
you can see the Vista window border around the edges of the second image; even though the program automatically stretched to fill the screen, it won’t return to a true fullscreen mode no matter what I do. Like I said, not a huge deal–I’ll just try to avoid using the minimize button in the future–but perhaps it’s something that the support team could look into.