I've tried the new Poisson shader from A7.66 to blur my shadow. But if I activate a graphics card controlled Anti-Aliasing the shadows are not visible any more.

Only if I set the graphics crads Anti-Aliasing settings to "let application decide" it works. I have an ATI Radeon HD 3800. The problem happens to all kind of shadows (decal, stencil, accelerated) so it have to be a shader problem.

Any ideas?

can you provide a small test app so we can try with NVIDIA cards?

I don´t know why it isn´t visible but I think that the blur is a postprocessing shader? If that is the case, you can´t use antialiasing with it. One approach would be rendering into rendertargets with a very high resolution and then to sample it down or the alternative which is probably a little more usable, but also harder to do is to use an edge detection shader and to blur the found edges with a simple blur shader...
But I guess that this isn´t really what you want.
You can find some more information about it in for example this article: http://http.developer.nvidia.com/GPUGems2/gpugems2_chapter09.html
There is also a little information about it in some of the shader X books.