To me, what's exciting & innovative about SO Careers is its integration w/ the high-quality, high-bandwidth information conveyed by SO reputations---not how SO executes the job board itself.
The primary problem in hiring developers isn't measuring ability (although that is of course an important problem) but in sourcing candidates. If SO can make SO Reputation the standard way of pre-filtering candidates, then there's certainly potential in that, but it doesn't seem to be what SO is focusing on.
To get quality, you need good filters; filters in the form of what types of people who will even see the listings before applying and good filters for evaluating the applicants once they do apply. Most systems fail on both accounts because they target a mass audience of all job seekers (the dreamers, not just actual programmers) and the resumes they submit are easy to trump up and can be expensive to validate.
The cool thing about Stack Overflow is that the community is great for both types of filters. At the first level, basically everyone who is a programmer is coming to the site regularly and also conversely, for the most part, programmers are the only people coming to the site. Second, participation in Stack Overflow (to the extent that they actually contribute content) leaves behind a great peer reviewed bread crumb trail of their actual experience and programming knowledge. Therefore, this content is a great thing for programmers to be able to include and feature on their Stack Overflow Careers profiles and also good value add for prospective employers evaluating candidates.