It's been around for ages perhaps, but for a long time has not caught on. Most of the time when I search google for a question that might be appropriate for programmers.stackexchange.com, I wind up with links to locked topics on stackoverflow instead. None of them even include references to SE:
programmers. The difference in site volume is telling as well. Quantcast gives stackoverflow 11.4 million uniques per month, while programmers.stackexchange is only 400,000.
The distinction seems clear from 60,000 feet but sorting edge cases seems to depend largely on capricious and arbitrary moderation. And practically speaking, in many cases you would actually get better answers on stackoverflow, but for the oppressive moderation.