Okay, in principle I was finished two weeks ago but I'm incredibly busy these weeks. Here is what I got so far:

45°-angles seem to be smoothed quite well.

high contrast edges are smoothed better than low contrast ones; also, non-45° edges are somewhat ragged. also note the weird artifacts.

again, weird artifacts, but it does look smoother.

The good news is, it works! It is extremely cheap on resources (render time some miliseconds on a GF 8600m GT) and independent of scene complexity.
I have to work on the artifacts, though, and I'm still curious on other implementations. It all depends on the edge detection algorithm you use, propagation and composition is quite straightforward.

Joey.