The legal issues regarding copyright, trademark, and intellectual property are 
very complicated and multifaceted, but for writing fanfiction there are only a few
things you need to know:

By strictest interpretation of the law, writing a work of fanfiction is not legal.
Xeroxing a comic strip and putting it up in your office is also not legal.  Please
note that plagiarism/copyright violation is a civil matter - you are not committing 
a misdemeanor or anything like that.

Practical application of the copyright law, however, only makes a difference
when somebody is making money.  None of the fanfiction writers I know about
are making money with their stories, so we are all pretty safe.  Those folks who
write "don't sue me" on their stories are either making a joke or being needlessly
concerned.

A company (say, Viz) could take legal action against you, but could only really
get anywhere if they could establish that you are either making money with their
property (if you sell advertising space on your fanfic page, sell copies of your story,
use the story to draw people into a site that sells merchandise, etc.) or that you
are inhibiting their ability to make money with their property (giving away stories that
they're trying to sell, reducing interest in their work with unjustified negative 
portrayals).

I'll give you an example of the latter point.  George Lucas did not mind that there was
a singer out there calling himself "Luke Skywalker," even though that character was
his property.  However, when he discovered that the singer in question was performing
highly objectionable/explicit music, he was able to get a court to agree that this singer
was damaging the reputation of his character, and thus the overall marketability of his
products.  But, if I'm not mistaken, he did not get any money out of it -- he just got 
the singer to stop using the name.

And this is the crux of the matter for fanfiction writers.  Even if you wrote a Ranma story
which Viz found extremely objectionable for some reason, and Viz was able to 
demonstrate that you were damaging the reputation of their products, the most they
could really do would be to make you take it off your page.  Again, if you are using
their products to actually make money, it becomes a different matter.

As for the matter of characters of other fanfiction writers, there is so little viable legal
grounds for voicing a complaint one way or another that it's not really worth mentioning,
since (again) money is not generally involved.  The issue is really a matter of honor,
integrity, and politeness.  These matters, to me, transcend law.  The fundamental 
principle here is that if somebody writes a story or creates a character, it is morally
wrong to rip them off.  If you love somebody's character(s) and want to use him/her/it/them,
just ask.  The likelihood is that they will give you their blessing.  If they do not, then you
are not legally prohibited from going ahead and writing what you wanted to write, but you
should be ethically inclined not to do so.  

(I'm not a lawyer, this isn't legal advice, if you want real legal advice, talk to a lawyer.
Or a judge.  That would do the trick too.)

Grayson Towler
grayson@rigroup.net

http://www.rigroup.com/~grayson/relentless