Additional Observations on "task bar doesn't autohide"
-- At least 90% of the occurrences (on my system) are caused by a particular running application wanting attention. One clue is that the needy program may change its taskbar icon to have a brighter background color than usual. So the first thing to do when the problem occurs is to look for a "bold" taskbar icon, click it to bring that application window to the front, and take care of whatever that application is asking for. This is a "feature" not a "bug"

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-- Remember that even after you fix the underlying problem, the taskbar may remain displayed until you click on the desktop.
-- If the above doesn't identify the problem, minimize all applications to see if there is a small system or application notification window hiding back there somewhere. Such a window may also keep the taskbar displayed - dismiss it and the taskbar will hide.
-- An application that wants attention may also display its tiny icon in the "Notification Area" (the group of tiny icons at the right end of the taskbar), sometimes also displaying a balloon pointing at that icon. This might keep the taskbar from autohiding. Many programs still do not fully conform to Win7 standards in this respect. I've seen at least one older program where I had moved its icon to the "hidden" part of the notification area, and that icon did not jump down to the visible notification area when attention was wanted. This may also keep the taskbar displayed, with no visible clue to the problem. To investigate this, click the little "show hidden icons" arrow in the Notification area and mouseover each of the "hidden" icons. Normal icons will simply identify themselves with a popup message. An application needing attention may identify the problem in this popup. This is a flaw in the application, not in Windows 7.
-- The quickest and simplest fix when all of the above fails to solve the problem is to open Task Manager, kill the explorer.exe process, then re-start it. Task Manager makes this easy: In Task Manager, after right-clicking the process and clicking "end process", click File/New Task and enter "explorer" (you don't need the full path).
-- The application that most frequently causes the "no autohide" problem on my system is Adobe Photoshop Elements Editor, v.8. If I leave it running but minimized, it's almost certain to cause the "no autohide" problem within a few hours. It does change its taskbar icon to "bold" when this happens, and I now know to look for that. The odd thing is that the program never seems to actually want anything specific - it, like my cat, just periodically craves attention. Bringing up its window, then minimizing it again is all that's needed to fix the problem.
/Dave