I just got a mail from BenQ saying, that although 854x480 is the native resolution of the beamer, it doesn't support it, at least not on the DVI-In.
Their excuse was, that PAL movies come in *x576, so that some scaling would occure anyway, so running in the native resolution wouldn't increase picture quality.
But apparently they didn't think that anyone would be doing other things than watching movies with the beamer....
Now my only hope is that they adress this in a firmware update. I taunted them a little bit with the fact that nVidia is at least trying to do something.