This passage from Justice Alito's majority opinion (pdf) is not particularly reassuring:
This does not mean that there are no restraints on government speech. For example, government speech must comport with the Establishment Clause.
Precisely. But how does Pleasant Grove's decision to display the Ten Commandments without displaying monuments to competing religions comport with the Establishment Clause?
Justice Stevens' concurring opinion also sidesteps the issue.
For even if the Free Speech Clause neither restricts nor protects government speech, government speakers are bound by the Constitution’s other proscriptions, including those supplied by the Establishment and Equal Protection Clauses. Together with the checks imposed by our democratic processes, these constitutional safeguards ensure that the effect of today’s decision will be limited.
Telling us that Pleasant Grove is "bound by" the Establishment Clause doesn't explain why Pleasant Grove isn't violating that Clause by giving preference to certain religious monuments over others. And relying on the safeguards of democracy offers little comfort to adherents of a minority religion. How many voters will want to throw the bums out of office who preferred the Ten Commandments to the Seven Aphorisms of the Summum religion?
Justice Scalia (who does not lack for boldness) boldly stepped into the Establishment Clause debate in his concurring opinion. Joined only by Justice Thomas, the opinion argues that the Ten Commandments are of "historical" importance, as is the monument itself, given that it has been on display for 40 years. It is true that the Seven Aphorisms of the Summum religion are not as old as the Ten Commandments, but does that make the Seven Aphorisms historically unimportant? And since no monument to another religion will be tolerated in Pleasant Grove's parks, none will have a chance to stand for 40 years. The fact that municipal intolerance to religious diversity has existed for 40 years hardly seems a justification to continue that intolerance.
Justice Souter's concurring opinion acknowledges that the question is not so easily answered:
The interaction between the “government speech doctrine” and Establishment Clause principles has not, however, begun to be worked out.
Justice Souter's more nuanced view provides hope that future decisions will not give a blind pass to governmental speech that favors some religions over others.