Honestly, if your family tree takes you back to Europe, there is a outstanding chance that you have a Jewish leaf or two in the tree. In general, it will be the mother who was Jewish and usually the matriarchial side of the trees are not as strongly looked at as the patriarchial. My family name is very unique and I have done a lot of work in researching my family tree and have helped several others with their trees. The Jewish population has been prevelent in nearly every European culture and they have been marrying out of the religion for a LONG time not to mention the raping and pilaging that was routinely done in conquest of cultures for centuries.
It would not suprise me in the least if Hitler had some Jewish blood in his family tree. He certainly did not have the Aryan physical features that he was looking for in the perfect German.
Well, simply because it's printed in a book doesn't make it true; however, sometimes truth is stranger than fiction. It wouldn't surprise me if Hitler had some Jewish ancestry, as Vincenzo stated, but I doubt it was as blatant as an illegitimate grandfather. It's something I would like to do research for myself and make my own judgment.
I did not find either argument referred any legal documents so much as heresy third party information. Given Hitlers personality I would be wiling to bet one reason that he did not want any one poking around would be a history of mental instability. Far as being part jewish it would not be surprizing. I would have to dig around a lot before I would believe either story wholly.
I don't think it is so "stupid". There are quite a few points that make you wonder about Hitler's background. Let's start with his appearance, he did not look like the typical blond hair blue eyed Aryan that he tried to instill on people's minds, he looked all the opposite with a rather large nose, dark features and less than average physique.