An atheist sign proclaiming that "Religion is but myth and superstition" has been included in a holiday display on public property in Olympia, Washington. The display at the Washington state capitol building also includes symbols put up by religious groups.
I'm not convinced that this sign is a good way to promote atheism. Passers-by who are not atheists themselves are likely to find it more offensive than persuasive. As a legal matter, however, I think that atheist groups should have the same rights to put up displays on public property as religious groups do. I don't object if theists are allowed to put up creches, menorahs, and so forth on public property so long as agnostics, atheists, and others are accorded similar privileges.