This has been in OSX for ages, as luxun said, it used to be visible when you enabled keyboard navigation for all controls.
Basically, the Return key performs the least destructive/safest action, spacebar the most destructive.
Eg: In case of a file save dialog, perhaps triggered by closing an application with unsaved changes, return will save, space will either cancel the dialog or don't save the file, Esc will one of the remaining options.
Eg2: pressing the power button, in Lion it's a bit different as the spacebar toggles the reopen windows, but usually, the spacebar restarts the computer and return turns it off, escape will dismiss the dialog cancelling everything.
Not sure if it's a matter of safe/unsafe, guess it's the most obvious option, in the example of turning off your computer, the return key is more likely to be pressed and therefore will complete the primary action of the power button: toggle the computer on status.
It's actually very handy once you get used to it and expect all the dialogs to work that way.