I think a STackOverflow reference is GREAT! But then those of use that love StackExchanges are going to think that. :)
I do, in fact, list my participation as an interesting community involvement item on the end of my resume. I link to my profile and mention what areas I do particularly well at question-answering (since that's my main involvement around here). If you haven't done a ton of answering, you may also point out some particularly interesting work - like your example above.
A cover letter is usually a summary and doesn't get too heavy on the details. What you may want to say is that you are:
- passionate about solving problems by listening to the real needs of the people with who you collaborate
- focused on getting it right, and aware that small details can have a big impact
And then gently mention that one place you've practiced these skills is StackOverflow as can be seen from the details in your resume. I'm angling gently away from "I have a great attention to detail" as a term or general phrase to describe yourself, because my experience has been that "attention to detail" can cut both ways. Attention to detail is great when the details are crucial. But knowing when to see the big picture and gloss over a few details is also important. So phrasing it as knowing how to use details to get good work accomplished is quite a bit more powerful and moves the reader away from a vision of a guy who is counting the grains of sand on a beach while everyone else is trying to make a sand castle.
The other trick is covering the fact that your submission may or may not exist in electronic form in the company. A hip young company almost certainly will keep resumes primarily in electronic form. But a bigger company with a more old-school system may still be passing resumes by paper. Even in a big, well networked, computer-oriented firm, the last thing to be updated can often be the HR systems. So some poor manager may be looking at a really long URL or a URL that was a hyperlink, but is now pure text and he's wondering what on earth you mean by "Example Question"
If you really want to highlight work that is accessed electronically try:
Contributor to Stack Overflow - Brownish Monster at www.stackoverflow.com, particularly interesting work expressed in "Why are there random characters appearing in my decrypted text?"
That will make it relatively easy to get to your user profile. Also it's easy to keep this fresh - if you should have other interesting highly voted answers, they'll show up in your profile.