How important is compliance with the various accessibility guidelines when developing a website, especially if you don't know who will be using your site?
Using the HiSoftware Cynthia Says Portal, I actually checked StackOverflow and found that there are only a few warnings for Section 508 compliance, so I would assume that it is easier for the disabled to utilize Stack Overflow (although I didn't check anything other than the homepage).
I don't know about the commercial sector, but it is required for the government sector. There are generally reviews that any software project must go through to ensure the product is 508 compliant before it is released.
Also, when I would give demos of software, the first question from the audience would almost always be "Is your product 508 compliant?"
So, if you plan on doing business with the government, make sure that your software is compliant.
Sites designed for accessibility generally have better usability and search engine love.