You might have read recently the New York Times article (http://www.nytimes.com/2008/06/06/us/06license.html) about the South Carolina legislature voting unanimously to allow its residents the choice of purchasing an auto license plate exhibiting a cross over a stained-glass window with the words "I believe." This is a Constitutional issue into which many people also inject their ideas of who were the Founders of America.
Make no mistake, nearly none of the Founders (and later Lincoln and by far most all presidents) were what today would be labeled "Christian," even though some of them included references to God (most were Deists) in speeches and letters (particularly Washington, but surely not Franklin).
John Adams (born Congregationalist; converted to Unitarian, which rejects divinity of Jesus) thought that religion was actually good for the masses (his term) to help keep them in line, but not suitable for educated, Enlightenment, rationalist thinkers like himself (and Jefferson, with whom he corresponded in many letters). Ironically, Adams today might think license plates with crosses would be good for the uneducated rabble to help keep them in line!
Seems to me, though, that a cross on a license plate, since it is a free choice not required by government (although it is a governmental activity--here is the controversy); and that the practice does not establish a religion, since all religions are free to design their own symbols for which they can petition the State--then the cross-on-license-plate practice would be protected exercise, according to the First Amendment.
I give thanks to Washington, Adams, Franklin, Jefferson, and Madison who were "guided" (Adams disputes this; he said they used only their rational faculties) in setting up government for maximum freedom.
For my personal plate, I would like to design a symbol for a religiously syncretistic Pagan-Buddhist-Baptist, which I am (with atheist-rationalist proclivities)! Perhaps the Buddha sitting on a river bank reading the Sermon on the Mount (certainly the most radical of biblical scripture) by the light of the full moon on the summer solstice and surrounded by a swaying circle of bodhi trees.
What symbol would Adams design? Probably a facsimile of the U.S. Constitution--with a ratiocinating human brain perched on top.