John Lemon:

Copyright doesn't protect the idea of an author, just the actual TEXT.

Sec 102 Paragraph (b) - In no case does copyright protection for an 
original work of authorship extend to any idea, procedure, process, 
system, method of operation, concept, principle, or discovery, 
regardless of the form in which it is described, explained, illustrated, 
or embodied in such work. 

"Copyright does not protect ideas, concepts, systems, or methods of 
doing something. You may express your ideas in writing or drawings and 
claim copyright in your description, but be aware that copyright will 
not protect the idea itself as revealed in your written or artistic 
work. " - Taken from the Government FAQ

Section 106 contradicts this, as far as fanfiction is concerned:
>From Section 106:
"Subject to sections 107 through 120, the owner of copyright under this
title has 
the exclusive rights to do and to authorize any of the following: 
   <snip>
          (2) to prepare derivative works based upon the copyrighted work;
 <snip>"

  Since fanfiction could be considered a "derivative work", copyright law
applies.
Ergo: To make fanfiction for a certain author or series legal, you require
the permission 
from the author/authors.

Of course, I could be wrong, since I'm not a lawyer.. ^_^

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