Definitely a bug, and independent of the graphics board. On a friend's ThinkPad T400 the alt-tab window always stays on top; on my ThinkPad X201 is sometimes does and sometimes gets behind the window being peeked at (which is the bug); on another friend's
X201 it always stays on top (same specs as mine).
Trying to determine what makes it "break" on mine. I first thought that's how it was supposed to work, then saw it work the right way on the T400; while attempting to demonstrate the problem to the T400's owner, I realized it actually started working on
my X201. I initially contributed it to a hibernate - docking station - reboot cycle (reboot fixes it, coming out of hibernation on the docking station breaks it), but I just rebooted (it started misbehaving again today), but that did not fix it.
Will post something as soon as I determine the right sequences to break and to fix.