The threshold for change seems to be when people who know the correct form opt not to use it, because it seems too fussy. I would say that's happened now with trailing prepositions and split infinitives. We aren't near that threshold yet with "begging the question" though. We may never be, because it's still needed for its original meaning.

OTOH, nearly each time there's a post with the incorrect usage, there's also a comment or two of correction.

Which begs the, er, which raises the question, how do you know when a certain usage has become more common? You can't just look at your own social circle (or HN, or whatever) and decide that's how everyone uses it.

1.6 million

http://www.google.com/search?ie=UTF-8&q=%22raises+the+qu...

38.9 million

for what those numbers are worth