In Sicily, the Mafia helped the Allies with the invasion. Mussolini had hit the Mafia quite hard (the only good thing he did) and helping the Americans (which of course they were already well familiar with) suited them fine.

They are also traditionally ultra conservative, and therefore could be relied on not to get involved with the communists Partigiani, in fact to keep them in check.

I am not sure what serious historians think of Tim Newark, but here's a book of his on the matter http://www.amazon.com/Mafia-Allies-Americas-Secret-Alliance/dp/0760324573