Dispatches From The Culture Wars has an interesting discussion on whether Indiana's In God We Trust license plates could be considered a violation of the establishment clause. Probably not, suggests one observer, saying it is "analogous to the same motto on the currency, which the courts have ruled is not a violation of the establishment clause." On the other hand, other specialty plates cost motorists extra, while the religious one costs only what standard plates would cost. Therefore:
By treating this message plate with an explicit religious endorsement differently than other plates, the message is that the state prefers that message over all the other optional messages available. One could even argue that the message is being subsidized, indirectly, by being free as opposed to other optional messages available.
A friend's father went to get new plates and was just given the In God We Trust plates without being asked if that's what he wanted. If a lot of this has been going on, that could be one reason the plates have become so popular, and the state might have a little trouble if this ever goes to court.
UPDATE: In the comments, Kevin Knuth points out that there has been discussion on an Indianapolis blog about the BMV handing out In God We Trust plates without being asked. Here is the link.