I've been seeing this on Windows 2000. I can't remember if my XP machine at home has this behaviour since I'm rarely running enough programs to see it.
Anyway, if I open several windows (enough that the task bar buttons are reduced to just the icon, one letter, and a "...") and then close a few of them, the buttons stay the same size instead of resizing themselves to fit the newly freed up area. Once I close a few more, Windows finally gets around to resizing them to take up the whole area again.
Is there any way to tweak this behaviour? It'd like it so that Windows recalculates the task bar button sizes everytime a window is closed...
