I would imagine that Flemish speakers would have a problem understanding Plattdeutsch since the pronunciation of the Dutch overtones in the language is very different from Flemish. I also notice in it's written form it seems to have huge differences from either German and Dutch - so I see what you mean by it being a separate language!

It also seems to bear quite a big resemblence to Danish, (although I don't speak Danish, I've heard it very often - any Mudcat Danes out there would like to comment?)

Graeme