In the 1960s, the Supreme Court attempted to define pornography, i.e., something not protected by the First Amendment, in a series of cases. The justices would huddle in the basement of the Court and review dirty films to see what was ok and what wasn't. Said Justce Potter Stewart, "I can't define pornography, but I know it when I see it."
That individualized level of review comes to mind when seeing the Court's decisions today on religious displays. In two separate decisions, the Court held that a framed copy of the Ten Commandments on display in a Kentucky courthouse is unacceptable, while a monument on the Texas capitol grounds is ok. The cases were decided 5-4, with Justice Breyer the one who switched his vote in the two cases. In the Texas case, Chief Justice Rehnquist, writing the plurality opinion (i.e., for four justices -- himself, Scalia, Kennedy, and Thomas), stated that the fact that the monument had been on display for 40 years without challenge was the determining factor.
Breyer cast the decisive vote, so that his opinion is much more legally significant (a fact that seems to be overlooked in the articles I've read). He found that where the monument was located (in a park as one of 17 monuments and 21 historical monuments), suggests State intended a non-religious, "moral message--an illustrative message reflecting the historical 'ideals' of Texans--to predominate." Breyer focuses almost wholly on intent in distinguishing the Texas case from the Kentucky case, stating that "the short (and stormy) history of the courthouse Commandments' displays demonstrates the substantially religious objectives of those who mounted them, and the effect of this readily apparent objective upon those who view them."
Still, the cases refused to lay out general guidelines on when it is acceptable for the government to display the Ten Commandments. Instead, they found that, in the words of Rehnquist in the Texas case, "No exact formula can dictate a resolution in fact-intensive cases such as this" (Breyer wrote something similar). This parsing of the minutely different situations puts lower courts in a difficult position -- odds are, the Supreme Court will need to hear a number of additional cases before a clear pattern develops on which the lower courts can rely. Moreover, if a justice or two steps down in the next few days, as is being rumored (Rehnquist &/or O'Connor), a pattern will be unlikely to develop for quite some time. In the meantime, in deciding when a display is acceptable, I guess they'll know it when they see it.