http://www.mactricksandtips.com/2008/10/use-tab-to-switch-between-dialog-buttons.html

The one single “bug” on Mac OS X by far is the ability to switch buttons in dialog boxes. For example I mean using Tab to go between OK, Cancel, Don’t Save etc. For a long while until today I had just got used to using the mouse. Although you can use some shortcut keys for Save, but these aren’t always reliable. Windows has the ability to tab through button options. Why not Mac? Well you can and it is blindingly obvious.
To enable you to tab through dialog boxes go to System Preferences > Keyboard & Shrotcuts. At the very bottom there will be an option saying about full keyboard access. Check the radio button that says “All Controls”.

Now when you find a dialog box (a new file in TextEdit is quickest), when you hit tab you will be able to move through the dialog box options. Very neat. As the option at the bottom states if you hit Control + F7 you will be able to change this setting, although I doubt would have to.