Although, I've always preferred the Apple system of "Destructive"......"Neutral".."Constructive"

So that destructive operations are on the far left, isolated from non-destructive operations on the far right.

Yielding,

  [Don't Save]          [Cancel]  [Save]

Also, in a window title bar (more classical than OS X now),

  [Close]               [Minimize/Shade] [Zoom]

Any of the buttons grouped on the left side should be considered "destructive actions", and any actions on the right side should be considered "safe" or "non-destructive actions". However, this isn't the whole story. In cases where the main, intended action is destructive, the primary action verb should still be on the right, but the [Cancel] button should be the default enabled button (what Return key does when pressed)—e.g., emptying the Trash.