Eric,
You asked about addiction and the brains reward system. There is a fascinating article in the newest issue of "Brain & Mind" on just this subject. (Complete with videos of rats giving themselves electric shocks in order to receive artificial stimulation of their limbic system!) I highly recommend it to everyone with an interested in the way their brain works:
REWARD DEFICIENCY SYNDROME
by Jorge Martins de Oliveira, MD, PhD
http://www.epub.org.br/cm/n08/doencas/drugs/sindrome_i.html
Surprisingly, (to me, at least) they link the syndrome and the gene responsible for it to compulsive gambling as well:
"Those who research on this subject point out that the risks and
expectations associated with this vicious condition, give rise, in the
obsessive gambler, to a stage of euphoria comparable to that produced by the
inhalation of cocaine (Ref.6)."
-Prof. Tim