[Semi OT]
Thanks for the reply and links, Jim. Did a bit of reading and seems the story is a bit more involved for the "common" addiction as well. Although DA is thought to be involved in the rewarding/reinforcing effects (a whole separate discussion on the difference between the two) of all drugs and "natural" rewards, this is sometimes not a direct activation of the system as in psychostimulants (cocaine, amphetamine). Opiates, e.g., heroin, are thought to activate the DA system by acting on GABA terminals (inhibiting release of GABA), as well as having a DA-independent reward mechanism.
Although GABA agonists, like benzodiazepines, certainly have abuse potential, it is not considered particularly high (it is only Schedule IV in the controlled drugs scale). The addiction potential is indeed not well understood, including the fact that it is not easy to have animals self-administer GABA agonists (although physical dependence readily develops), but DA might be involved, as usual. BTW, alcohol rewarding effects are thought to be mediated by its effects on GABA(A1) receptors, and alcohol is most definitely addictive...
Lunesta does seem less addictive with stronger clinical data plus it could go generic by mid 2012 (the 30-month stay expiration).
