Comment Google to the Rescue (Score 1) 513
Google results with "Wilhelm" = 2060 results.
With so many sources agreeing that "Wilhelm" is one of his first names it must be right!
So, Mr. von und zu Guttenberg: You better get your passport checked, there seems to be one name missing on it!
BTW if you can read german you can find a anonymous blog entry from the guy who added the "Wilhelm" to Wikipedia.
On that page I loved the quote from the "Süddeutsche" newspaper which translates into something like:
"...and his ten first names. Somtimes Guttenberg lists them. If you really ask him: Karl(1) Theodor(2) Maria(3) Nikolaus(4) Johann(5) Jacob(6) Philipp(7) Wilhelm(8) Franz(9) Joseph(10) Sylvester(11)"
(emphasis & numbers mine) So they knew he had 10 names, but never bothered to count the names they copied from Wikipedia)