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This is really an issue for the KDE developers rather than for you, but under the version of KDE I got from kde.org for RedHat 5.2, there was an option to allow other winders to be raised above the KDE panel. That option seems to be missing from the version of KDE shipped with RedHat 6.1, so there's no way to prevent the panel staying on top. I find that frustrating; often I want a window to fill the screen completely (e.g. for a presentation or for doing graphics editing), and with the current version of KDE I can't do that. (I'm probably just going to figure out how to stop the panel from starting when I log in, and not use it.)
Right-click on a blank space in the panel, and click configure. Select the Options tab and turn on the "Auto Hide Panel" and "Auto Hide Taskbar" options.
`Auto Hide Panel' isn't the same thing. That shrinks the panel to a few pixels at the bottom of the screen, but that shrunken panel still still stays on top of any other windows, which means I can't make a window occupy the full size of the screen, and I can't pass mouse-clicks to applications at the bottom of the screen. (I understand that this is a matter of personal preference, but it's a bit frustrating that the previous version worked the way I wanted it to and this version doesn't.)