Beryl Adcock, an estimable editor whom I had the good fortune of meeting at the first American Copy Editors Society conference in 1997, sends an inquiry: "Seeing a lot of 'different than' in news stories. Is this another old 'rule' we needn't be concerned about anymore?"
The Associated Press Stylebook says flatly (and wrongly) that different "takes the preposition from, not than," and the point is one over which some people's pulses have been racing for decades.
But Merriam-Webster's Dictionary of English Usage says calmly that "different from is the most common and is standard in both British and American usage; different than is standard in British and American usage, especially when a clause follows than; different to is standard in British usage but rare in American usage."
All right, folks, show's over. Go home.
You want more?
MWDEU has another full page beyond this, explaining the origins of the objections to different than, which "has become a favorite topic of 20th-century comment,"* in part over the question whether than can be considered a preposition, but holding stoutly that all three forms have been in common use for centuries.
Bryan Garner prefers different from most of the time: "Than implies a comparison, i.e., a matter of degree; but differences are ordinarily qualitative, not quantitative, and the adjective different is not strictly comparative. Thus writers should generally prefer different from. ..."
But, he adds, "Still, it is indisputable that different than is sometimes idiomatic, and even useful, since different from often cannot be substituted for it--e.g." 'This designer's fashions are quite different for men than for women.' "
So, Beryl, the "rule" that one must always use different from is not rule at all, but an oversimplification about usage. Different from is common, different than sometimes more appropriate. Don't allow the AP Stylebook or other manual turn you into a mechanical editor.
*Ah, those long 20th-century nights of comment at the bar over a pipe and a pint, with the different to/different than issue still unresolved at last call. It was a different time.