Everything regarding fair use is definitely in a gray area and is highly dependent on the issue at hand. However, from what I've seen, archiving is allowed as long as redistribution doesn't enter the picture.

One famous case is this one: http://en.wikipedia.org/wiki/L.A._Times_v._Free_Republic

The judge held that "while defendants' do not necessarily 'exploit' the articles for commercial gain, their posting to the Free Republic site allows defendants and other visitors to avoid paying the 'customary price' charged for the works".

Under US copyright law (Title 17 Section 107), "fair use" is defined as "purposes such as criticism, comment, news reporting, teaching (including multiple copies for classroom use), scholarship, or research", and then only upon consideration of these qualifications:

(1) the purpose and character of the use, including whether such use is of a commercial nature or is for nonprofit educational purposes; (2) the nature of the copyrighted work; (3) the amount and substantiality of the portion used in relation to the copyrighted work as a whole; and (4) the effect of the use upon the potential market for or value of the copyrighted work.

http://www.law.cornell.edu/uscode/text/17/107