Being able to use Tab to navigate buttons requires the appropriate setting in System Preferences > Keyboard > Key combinations > All controls. (Not sure about the actual names, my OS X interface is not in English.)

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Normally, yes, but I recently discovered that in the Finder in Leopard (10.5.8) I can navigate the buttons of such dialogs with Tab and Space, even though I keep Full Keyboard Access set to “Text boxes and lists only”! (This is not the new Cocoa Finder in Snow Leopard, and this is not the case for me with other apps. So it must be extra functionality deliberately added to the Finder for convenience.)

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Wow, that's actually sad: someone at Apple finally got the clue and then implemented it in Finder rather than in some OS X library... :-(

(If only every dialog would respond to Command-letter shortcuts I'd already be happy...)

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