New York has a good article about the New York State Bar Association doing research on what is and isn't ethical for lawyers to do some research on Facebook for a case.
It seems, actually, like most of the rules that work in the brick-and-mortar world can be fairly easily ported to Facebook -- if it's public, it's public, don't engage in relationships (i.e. friend) with an adversary or their lawyer, don't use false pretenses to gather information.
