It depends on where you are and who you're competing against. In smaller markets, or if you have unique skills, employers searching will be finding a small number of employees, and won't care about Stack Overflow participation. So for example if you know lisp and want a job in Austin, you're competing against exactly one person, who only has 424 reputation, so you're pretty likely to get a call anyway even without StackOverflow reputation.
If you are competing against more people, say, if you're looking for a Java job in the Bay Area, it may work against you, but I doubt if it will make the site useless. In my own recruiting using Stack Overflow careers I found plenty of good people who didn't even use Stack Overflow and I still contacted them because their CVs were great.
PS when I search StackOverflow I look for lisp, OCaml, ML, Smalltalk, etc., not because we need them, but because we like to hire people who know them.
Joel, on a slight tangent...we've been seeing all sorts of Jeff and Joel bashing posts lately, which personally I don't get why people get so worked up.
If I had to take a guess though, it would be that as you guys get more and more successful there's a fear that you will succeed in creating the number one method in the world for getting a programming job and it will end up discriminating against people who don't answer questions on StackOverflow or fit your model of a programmer.
That somehow you guys end up being the gatekeepers for defining what a good programmer is and how they get hired.
That's crazy fantasy extrapolation, almost too absurd to address, but I will anyway since you asked :)
There are 1492 CVs on StackOverflow right now, and maybe 9,000,000 programmers in the world. Even if we're monumentally successful we're still going to be only a tiny fraction of the hiring of programmers that goes on in the world... a TINY fraction.
I should also add that in the bizarre scenario that StackOverflow actually became a common way of getting programming jobs, it would be a metric ton better than the current system of Monster and Dice and emailing around Word resumes which are scanned in by stupid software that looks at keywords.
I think this post is on the money. You have to bear in mind that right now the "gatekeepers" you're referring to are mostly completely non-technical HR people who might as well be using an Ouija board for all their proclivity finding good programmers. One can argue the merits and demerits of judging someone based on their Stack Overflow participation, but it can't be worse than the current reality.
If you are competing against more people, say, if you're looking for a Java job in the Bay Area, it may work against you, but I doubt if it will make the site useless. In my own recruiting using Stack Overflow careers I found plenty of good people who didn't even use Stack Overflow and I still contacted them because their CVs were great.