Everson v. Board of Education, 1947. Justice Hugo Black wrote...
"The 'establishment of religion' clause of the First Amendment means at least this: Neither a state nor the Federal Government can set up a church. Neither can pass laws which aid one religion, aid all religions or prefer one religion over another. Neither can force nor influence a person to go to or to remain away from church against his will or force him to profess a belief or disbelief in any religion. No person can be punished for entertaining or professing religious beliefs or disbeliefs, for church attendance or non-attendance. No tax in any amount, large or small, can be levied to support any religious activities or institutions, whatever they may be called, or whatever form they may adopt to teach or practice religion. Neither a state nor the Federal Government can, openly or secretly, participate in the affairs of any religious organizations or groups and vice versa. In the words of Jefferson, the clause against establishment of religion by law was intended to erect 'a wall of separation between Church and State.'" 330 U.S. 1, 15-16."
I sourced this quote from
http://en.wikipedia.org/wiki/Everson...d_of_Education
This wikipedia entry also has a little bit of historical context for the decision.
You have a legitimate question here, but before I respond I want to point out that the word "establishment" in the context of the bill of rights is a noun, not a verb. Its not in reference to establishing religion, but rather the institution that is religion. I realize this is a minor point, and that I'm kind of an anal grammar nazi sometimes, but it does make a difference and it is an important distinction.
The answer is that the symbol of the cross represents all denominations of Christianity, just like the star of David is a meaningful religious symbol to any Jew. Displaying the icon of Christianity could potentially be a violation of the establishment clause or federal law if certain criteria are met.