I finally can reproduce waht causes the intellisense to loose. I think this is definetely a bug !
Adding a static ressource to the page via blend results in adding the namespace to the header. After adding the ressource Intellisense is lost for the default (ms) namespace AND the designer does not show expand/collapse markers anymore. If one cuts the ressources xaml, the markers appear again and stay when re pasted.
I guess this is defintely a bug ?
Joe Robe