Well there are two types of rgba images - straight (aka unmatted) and premultiplied.
The three rendering types in blender are "sky", "premult" and "key".
"Sky" would be a rgb format, and "premult" and "key" are rgba formats.... premult would be premultipled and key would be straight (aka unmatted).
Basically in straight images, the intensity of the object's colours remain the same at their boundary, rather than fading to black, like in premultiplied images.
More info about straight vs. premultiplied rgba images
If you render a png or tga image with "key" and OSA and RGBA it should theoretically have an alpha channel that is smooth so that the opaque parts of the image have smooth boundaries too... (the aliasing of the edges' colours is removed due to the alpha channel)
But the don't have an alpha channel in the programs I tried - except when I viewed the PNG in the Windows XP picture viewer.... originally the image had a blue background that has an aliased boundary, but in the picture viewer it had a white background with a smooth boundary.
I think the button "Key" should be called "Straight", and if you press "Sky" it could which to RGB mode (if you were in RGBA mode) - and if you press "Premul" or "Straight" it should switch to RGBA mode.
You seem to have already answered your own question. The edges are not smooth because the smoothing lies within the alpha channel.
The thread's title wasn't meant to be a question - it was a statement... it's for educational purposes.
It also includes a suggestion in there for the interface. (That the name on the button be changed)
Hey it is truncated... the subject field let me type in the whole thing I wanted though.