Actually, it is not such a clear-cut violation. The following clause, and any other clause in the constitution, has many interpretations.
"Congress shall make no law respecting an establishment of religion..."
One way of interpreting this is that this Establishment Clause means that congress shall make no law that establishes a singular, national religious identity (United Christian States of America, for example). A second way is that Congress shall make no law showing preference of one religion over another. In Aronow v. United States, the court ruled, on this very issue, that "It is quite obvious that the national motto and the slogan on coinage and currency 'In God We Trust' has nothing whatsoever to do with the establishment of religion. Its use is of patriotic or ceremonial character and bears no true resemblance to a governmental sponsorship of a religious exercise." According to the official (this may not be your interpretation, but it is the government's for now) interpretation, the Establishment Clause only prohibits the government from establishing a national religion.
As the motto "In God We Trust" does not establish a national religious identity, this motto is not, in the eyes of the officially held interpretation of the constitution, in violation of the First Amendment.