As it turns out Speedlite 580EX II produce struly even frame coverage from 50mm flash zoom setting and up.
The wide angle positions have a noticeable light falloff:
All light sources obey the laws of physics and have inverse-square fall-off so its not really realistic to expect a 2" x 3" source to cover a flat wall evenly because the far edges are much further from the flash head than the center.
But if the wall was curved, equidistant from the flash the light would be more even because the light radiates from the source in an arc. One of the tricks for lighting a group is to arrange the group around the flash in the center in an arc or chevron pattern rather than straight rows to keep all the faces equidistant to the light.
Better options when even room lighting are to use a reflection-diffuser on the flash to spread the light more evenly or bounce the light off the ceiling which makes the point of origin (and fall off rate) equidistant in all parts of the room.
The built-in white card isn't intended as a modifier to spread the light. It is called a "catchlight card" and is intended to create a "catchlight" reflection of the card in the eyes when ceiling bounce is used.
Great info Chuck! I just want to point out that I was not trying to show that Speedlite 580EX II is not good enough, or that the catchlight card does not provide sufficient frame coverage. I was trying to show (and see for myself) the way they are .
.