There are at least three specific situations where including Stack Exchange participation can be valuable for your career or for landing a job; however, keep in mind that this is only valuable to a certain extent.
1. Project Management Certifications
How Can I Maintain My PMP Certification and Earn PDU's In an Inexpensive Manner?. Here, Ashes999 writes:
I posted about this before; one of the best free ways to eearn PDUs (in category C -- self-directed learning) is to spend time on the PM Stack Exchange website (i.e. this site). You can use RescueTime to track hours; I've claimed around 4-5 PDUs already from this method. (You can read more details in my blog post here.)
Other than that, PDUs generally cost about $10 each (that's the most cost-efficient I've seen) if you purchase materials.
In short, if you're a professionally certified project manager, you can use Stack Exchange as a way to help maintain your certification.
2. Microsoft MVP Certification
We have a few very high rep members of this site, such as Jon Skeet, who are also a Microsoft MVP. While I don't have all of the details about this program, providing professional leadership in the Internet community is one of the requirements.
From the Microsoft MVP Website "Contributions" page:
MVPs reach out and contribute to online and offline communities through a broad range of channels. These include online forums such as Microsoft Answers, TechNet and MSDN; wikis and online content; conferences and user groups; their own podcasts, Web sites and blogs; and articles and books.
3. Stack Exchange
When Stack Exchange hires from the community, sometimes they look at how active a candidate is in the Stack Exchange community. For instance, Kevin Montrose, the author and maintainer of the Stack Exchange API, was hired because he had already started working on an API, and Stack Exchange needed someone to build an API.
In Kevin's case, highlighting his Stack Exchange contributions marked him as a top candidate for a developer position in Stack Exchange.
4. What about other employers/industries?
Now, with these examples in mind, don't get too carried away. Many employers realize that Stack Exchange can be a bit of an addiction, so focusing too much on this may actually backfire on you. If I hire you for a job, I want to know you're dedicated to my organization, not Stack Exchange, so be careful how much emphasis you put on this. Don't overdo it! ;)