When you select text in Office 12, the MiniBar appears "ghosted" above the text you selected. If you move closer to the MiniBar, it fades in and becomes a miniature toolbar you can use to apply Bold, Italic, Font Size, Color, etc. As you move the pointer away from the edge of the MiniBar, it fades away to nothing. So, if you don’t want to use the MiniBar on selection, just move your cursor a few pixels away and it dismisses.
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There is also a short movie showing the Mini-Bar in action, worth checking it out. Office 12 is looking like the best release of the suite to date.