A previous article explained the history of jurisprudence around this issue, which basically held that it's OK to have a portrayal of the Ten Commandments in among a whole bunch of "legal history's greatest hits" -- the Hammurabic Code, Solon of Athens, the Magna Carta, stuff like that -- but it's not OK to display it alone. Which seems reasonable to me. Too bad the Xian Right has been so successful packing the courts with wing-nuts like Scalia, who are all too happy to start dismantling that rule.
I think what I find most interesting about public Ten Commandments cases is how eager religious people are to claim that such monuments are secular, not sacred, despite their very obvious, borderline-idolatrous reverence towards such icons. They're so eager to get their way that they're willing to publicly compromise their beliefs, despite claiming that their beliefs are what motivates them in the first place. It's very peculiar.