I looked for the full text of the document that details the words of the Crown Prince about Ahmedinejad, but it doesn't appear that the Times has published it yet.
Update on November 29: a commenter provided the reference - it's in the July 23, 2009, cable from the Abu Dhabi embassy (http://cablegate.wikileaks.org/cable/2009/07/09ABUDHABI754.html), in the section entitled "Afghanistan - Neighbors Not Doing Enough." I missed seeing because of that headline. Here's the full text:
MbZ criticized other regional leaders for playing both sides and for "dating" Iran. MbZ compared the current situation to pre-WWII Europe saying, "Ahmedinejad is Hitler," and neighboring capitals believe erroneously that they can prevent Iranian retaliation by playing nice or signing agreements with Tehran. "They think the are backing the winning horse," MbZ explained, emphasizing that if they think that by appeasing Iran they will avoid Iranian retaliation "then they are seriously mistaken, Sir."