What's the difference between "blond" and "blonde"?

The usage note under blond in the NOAD entry is explicit:

The spellings blonde and blond correspond to the feminine and masculine forms in French. Although the distinction is usually retained in Britain, American usage since the 1970s has generally preferred the gender-neutral blond. The adjective blonde may still refer to a woman’s (but not a man’s) hair color, though use of the noun risks offense ( : See that blonde over there?): the offense arises from the fact that the color of hair is not the person. The adjective applied to inanimate objects (wood, beer) is typically spelled blond.

Blonde/blond is an example of English retaining a written French gender distinction which is not pronounced in English (though they can be distinguished in oral French). Another is fiancée/fiancé (which also sound the same in French). So in the following, the spelling is correct even if the sentiment may not be.

A blonde, a brunette and a redhead walked into a bar. The barman said, "Is this a joke?"

About the British usage of the words, I have found the following note, in the Oxford Dictionary of English 3rd edition.

The alternative spellings blonde and blond correspond to the feminine and masculine forms in French, but in English the distinction is not always made, as English does not have such distinctions of grammatical gender. Thus, blond woman or blonde woman, blond man or blonde man are all used. The word is more commonly used of women, though, and in the noun the spelling is typically blonde. In American usage the usual spelling is blond for both adjective and noun.

The NOAD has a note that says what already reported by @Robusto in his answer.