From Diablo Wiki
Like all modern video games, Diablo III will support multiple resolution sizes and dimensions.
The visible screen will scale to adjust to a player's monitor resolution, with widescreen viewers seeing more of the visible area to both sides.
The shifting nature of the resolution can be seen when comparing two different screenshots with interface windows open. Note the width of the space on the sides of the character in these two shots; the older shot is on top, with the 4x3 dimension resolution.