Apparently a lot of the rat addiction studies are flawed, in that they keep the rats in cages without social interaction or other forms of entertainment, so whatever drug/electrical stimulation the researchers provide is their only possible source of pleasure. Rats kept in more stimulating environments are much more resistant to addiction. (Source: http://en.wikipedia.org/wiki/R...)
I recall reading recently that a similar effect can be seen in humans. People who can get their jollies elsewhere mostly don't become addicated to drugs.