http://www.straightdope.com/columns/read/2295/is-different-than-bad-grammar

Cecil's Columns/Staff Reports
1

Given Cecil’s background, I’m surprised he did not simply refer the questioner to the (University of Chicago’s) Chicago Manual Of Style, which is a well-respected reference work for authors, proof-readers, sub-editors and others who work with American English, for a living.

My fifteenth edition (2003) advises: "different. The phrasing ‘different from’ is generally preferable to ‘different than’ [this company is different from that one], but sometimes the adverbial phrase ‘differently than’ is all but required [she described the scene differently than he did] "

I believe that other respected publications (such as the Harper Dictionary of Contemporary Usage or Webster’s Dictionary of English Usage) follow the same standard. Be aware that this advice applies only to American usage. UK usage would be ‘different to’. (I haven’t got my Fowler’s Modern English Usage to hand, or I’d confirm it.)

2

I have got my Fowler to hand (the original, not some inferior later edition), and his article on different from/to makes it clear that he approves of “different to”, but that “different from” is the predominant form in the UK. He cites unreasonable prejudice against “different to” as the reason for “different from”'s dominance.

What surprises me about the answer to this question is that Cecil suggests that “different than” is OK (here) in the UK. It most certainly isn’t: I’ve never heard anyone other than an American say it. To us it’s a quaint Americanism like “cellphone” for mobile phone.