Here is a NYT op-ed about automobile license plates. The writer —Stefan Lonce, who has a book on the subject of vanity license plates — distinguishes 2 forms of religious speech via license plate. First, there are the specialty license plates. South Carolina has introduced a Christian-themed plate that looks like this:
Second, there are vanity plates, and the states sometimes reject the letter/word combinations a driver requests. There is, we are told, federal court lawsuit about Vermont's rejection of a vanity plate that would read JN36NT (which is a reference to a Biblical passage).
It seems obvious that the individual expression in the form of a vanity license plate is the preferable to the state's provision of the religious message on a specialty plate. The "I Believe" specialty plate is almost surely a state endorsement of Christianity that violates the Establishment Clause. (There's no array of specialty plates for different religions and no atheist plate. What would an atheist plate look like?)
But everyone knows that what's on a vanity plate is chosen by the car-owner and doesn't represent the state's point of view. Vermont seems to be overdoing a concern about Establishment Clause and blundering into a Free Speech violation (though, according to Lonce, the federal district court approved the state's decision). The trouble with vanity plates is that at least some of them do need to be censored — there are always some people who want "F**KYU" — so it won't work to have individual choice as the only filter.
Lonce thinks the problem of censoring vanity plates can be solved by setting up a national data base, pooling the efforts of all the states to identify the offending letter/number combinations. An alternative is just to get rid of vanity plates altogether, but states make a lot of money selling them, and people want to buy them.