Quoth Terrence M Marks:

$ No.  Fanfiction, in any form, violates civil law.
$ Technically, the copyright holder can lose the copyright if it is 
$ not enforced.  However, I'd wager that no serious legal action will
$ be taken against your average author because a) we aren't posing
$ a serious threat to the copyright and b) suing fans is costly and makes
$ you look bad.
  then how do the Japanese doujinshi-writers get away with writing/drawing
  fanfiction and fanmanga?  I mean, there is a *huge* doujinshi industry
  of people who publish and sell fanmanga/fanfiction of characters that
  weren't theirs to begin with, and I don't see any of those circles 
  getting sued by authors.... in fact, a lot of those manga creators *want*
  to see their work being used in doujinshi because if your work gets 
  fanficked/fanmanga'ed, it means your work is popular enough among the
  general masses.....
  So all those people going to comike help violate civil/copyright law?

a confused hikaru.