Actually any of your original published work, whether in print or online, is automatically copyrighted (at least in the U.S.)  For something to go into the public domain, you either have to specifically renounce your copyright and place it in the public domain (i.e., add something like "I place this code in the public domain.  Do whatever you want with it."), or you have to let your copyright lapse (like Piano Sonata No. 8 by Beethoven).

If you write it at work, it's still copyrighted, but the copyright may be assigned to your employer either implicitly or explicitly.  The implicit ownership of copyright by your employer is a seriously gray area.  Of course it was designed to be that way by lawyers in order to keep other lawyers employed.

Many times when you write articles for magazines or other publishers, you sign a contract that specifically states you are assigning them the copyright to your work in exchange for $$$ or other compensation.  SSC, on the other hand, allows you to keep your copyright on your original work.  I do believe they ask that you not re-publish newly published articles anywhere else for at least 90 days (please correct me if I'm wrong on this).

The best thing to do is to check the terms before submitting articles, code, scripts, or even posting to the message boards, etc. to any site.

IANAL.