Really? I didn't know that, but he's the Language Guy, the Great One, so I should believe him, since he's got a lot more language experience than me (and most people).
So I clicked on his link to Language Log and saw the following explanation (excuse the long quote, but there's a lot of important info to share):
In 1672 an influential essayist called John Dryden published a critical piece called "Defence of the epilogue" which included a catalog of alleged faults in the writing of important recent authors. In that essay he called it "a common fault" to have a "Preposition in the end of the sentence". Notice, uncontroversially, the usage was common in the 17th century. That is because it was fully grammatical then, as it is now, and it already had been for centuries. Dryden even noted that it occurred in his own writings. He had no basis whatever for his objection to it...
...About a hundred years after Dryden expressed his opinion, Bishop Robert Lowth, in a grammar that became quite important, described the construction with the preposition at the front of the clause as "more graceful as well as more perspicuous", adding that it "agrees much better with the solemn and elevated style" --- though Lowth still made it extremely clear that it is normal in speech and "the familiar style in writing" (the style in which one writes I'm and couldn't rather than I am and could not). Slowly Lowth's view ossified in the writings of other grammarians. By 1800 several famous school textbooks expressed straightforward disapproval of the stranded preposition, and teachers began to teach generations of schoolchildren that it was wrong. In America (though much less in Great Britain) this belief survived from the 19th century into the 20th.
If you're wondering why he's so smart and knowledgeable, it's because he's a linguistics professor. So don't worry if your love of language is as "simple" as mine is compared to these linguistic heavyweights. The world needs us all.