As I am sure you could have guessed, rabid atheists are not satisfied with challenging the Constitutionality of the Pledge of Allegiance, they are looking at challenging the national motto, "In God We Trust," as well.[1] Some people seem to think there is not much chance of this challenge winning in court, but then, I am sure the same would have been said of the Pledge case before it went all the way to the United States Supreme Court.
[1] http://www.csmonitor.com/2005/0523/p02s02-uspo.html