No, it's not really a secret setting. It's an add-in for Office 2007 that will finally give IT admins relief from that one user who resists change in all possible forms. The one that reminds you every day that how they had everything the way they liked it in Office 2003 and why did you have to go and change it on them? Who cares that Office 2007 offers awesome new features, they want it to be exactly the same. Had enough? Just download the Ribbon Customizer, an Office add-in that is available in a free starter edition or a paid Professional Edition. Using the tool's built in "Classic UI Tabs" you can turn back time and make Office 2007's ribbon turn into Office 2003's classic look.
However, it would be wrong if I didn't mention that Ribbon Customizer can do a lot more than just enabling a legacy mode of sorts - the tool can also be used for redesigning the Office 2007 ribbon UI in any way you see fit. You can add, remove, and edit tabs, reorder them, create groups, move commands around, make customized tabs you can turn on and off, and much more. The Ribbon Customizer is available here if you want to download and try it for yourself.
(via Aaron Tiensivu)