Jim, if you want to offend every Plattdeutsch-speaker in sight, go on calling it a dialect! As far as I am concerned, it is my second language. Before c. 1600, it was not only the spoken but the written language all over Northern Germany down to the Rhine. This changed only with the spread of Luther's bible translation, which was in Oberdeutsch. I remember a history professor in Hamburg who'd come from the south and admitted he needed help with reading late medieval documents from his new area because all of them were written in Plattdeutsch. (Sorry for thread creep!)

BTW, Plattdeutsch is more closely related to Scots than German is to English, and I keep finding words which are very similar in both languages, e.g. but and ben (buten un binnen).