Now that you mention it, I've seen weird taskbar behavior too with the thing getting "unlocked" (so it's not always on top) spontaneously and having to re-lock it manually. I don't think this started happening until recently, maybe it's a side effect of some update or some software installation/configuration?
happens with me with the taskbar previews, have to restart to fix it.
I think I've seen this before -- isn't it just Aero that dies for some reason? You can just restart Aero from the control panel, you don't have to reboot the whole system.