AA on the shadow buffer? If that's what you mean, I don't think there's really one best way to resolve the depth values. And I don't think any way will give an appreciable amount of help to the softness.
There is indeed a very good way to use hardware AA to make depth-map shadows look better. When I get my hands on a X1k series card with high bit-depth AA, I'll let you all know about the results in a month or so...
Just as a note, AA on FP render targets theoretically works great with Variance Shadow Maps (http://www.punkuser.net/vsm/), although lack of support in the GL drivers has prevented me from testing that out...
Also note that *soft* shadows (i.e. correct penumbrae) are different from *antialiased* shadows. Naturally hardware multisampling may be useful for the latter, but not particularly for the former.
