That's a toughie...the argument that is being circulated is that based on the First Amendment:
(excerpted) Congress shall make no law respecting an establishment of religion, or prohibiting the free exercise thereof
Using this, the attacking view is that having the 10 Commandments (G-d's Law) in the on a government building (paid for by the taxpayers) is de facto endorsement of Judeo/Christian beliefs -- not quite the 'law respecting an establishment of religion' but close enough because the money that was used to construct the buildings and decorate them, money seized from taxpayers at gunpoint, includes athiests, Buddhists, and whatever other non-Christian belief you can conjure and that these peoples' monies should not go to paying for such things.
I'd just like to see this also applied to works of art that are used to decorate government buildings. I don't care if the place where I go to renew my driver's license has a sculpture in front of it. Instead of blowing money on art, hire more employees so that I may renew my driver's license in 15 minutes instead of an hour and 15 minutes.