Blender provides three different drawing modes to view alpha with respect to the image RGB channel. As shown in the following screenshot, these options can be seen in the menu panel of the UV/Image Editor. The RGB * A mode displays RGB with premultiplied alpha, the RGB mode displays only RGB information, and the A mode displays only alpha channel information in the image.

The RGB mode draws the RGB channel with "aliased" edges, since it doesn't consider the alpha channel.