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[Dave Britten]

The story I've heard - can't remember where - is that the X and O represent the typical Japanese marks for incorrect and correct respectively, thus cancel and confirm. A lot of US releases switch this and use X to confirm for some reason, but most Japanese releases use O for making selections.

The square is supposed to look like a piece of paper, or a menu, and is thus the menu button. The triangle resembles a cone and represents a field of vision, and is thus the "change view" button.