I think of markets in two main ways: robustness of information, and incentives.
On the information side, all the people who use StackOverflow have a great way to demonstrate their knowledge (or lack thereof). If an employer is interested in a candidate, they can view a (hopefully rich) history of their questions and answers to specific problems. This replaces any sort of quickie competence test you might administer in an interview.
And, you (the employer) can learn what others on the site thought of those answers. Imagine you’re a non-technical person tasked with hiring a technical one. You might have no idea how to recognize technical competence first-hand, but you get the benefit of seeing what a lot of other qualified people think. Crowd-sourced interviewing!
Which brings us to incentives. I notice that SO has more vigorous participation in certain programming areas (like Microsoft’s platforms and jQuery) while others that should be equally hot really aren’t (like Objective-C and Ruby on Rails).
There are probably sociological reasons for this — Joel and Jeff are on the Microsoft side of the fence and thus their audiences are too. The challenge for SO is to attract a wider range of communities — to go cross-platform if you will.
The careers site adds a “real” incentive for professional programmers to get involved with StackOverflow. The previous draw was simply community and reputation (and of course, getting your questions answered). And that’s all great.
Now programmers (of all religions) have a dollars-and-cents reason to get involved. If SO continues to grow, then your reputation on the site may be as valuable as anything on your CV. So one might expect some Lisp and Android people to find their way over there, too.