Currently, tthe fix doesn't work.

The Apple documentation says explicitly that the button becomes "default" when return is the key equivalent, it says nothing about the first responder.

Perhaps this should be theamable. For openstep style themes and the default theme, the return image is appropriate (like in alert panels), But for a cocoa or windows style theme, a blue color or a highlighted border would be needed.

On Mac, if you set the key equivalent of a Button to be "return", on loading of the NIB in the application it will get a blue highlight.
On GNustep it should get the "arrow" image, I guess.