http://www.derkeiler.com/Newsgroups/comp.security.misc/2003-05/0154.html
The use of the word "salt" is probably a reference to warfare in
ancient times, when people would salt the wells or farmland to make it less
hospitable. The Romans are sometimes supposed to have done this to
Carthage in 146 BC. In the context of passwords, a "salted" password
is harder to crack.
Apparently, there's no strong evidence even for the original "salting" of Carthage (http://en.wikipedia.org/wiki/Salting_the_earth) claim, but an interesting hypothesis nonetheless.