douchebag = 1,920,000 hits
douche bag = 799,000 hits
If the Google search is any indication, the dictionaries are making the wrong call. Enter "douchebag" into dictionary.com (which includes American Heritage), Merriam-Webster's site, or Webster's New World College Dictionary's site, and they'll politely suggest "douche bag" instead.
Of all these dictionaries, only dictionary.com acknowledges that the term actually describes a type of bag. The others list it only as an insult or slang.
I love English.