In Arizona, the state Senate approved legislation (HB 2046) sponsored by Rep. Ron Gould (R-Lake Havasu City) which would require the Department of Transportation to provide "In God We Trust" license plates if some organization pays the $32,000 necessary for design costs. The bill was originally for "Arizona Highways" license plates when introduced in January, but has been modified into a religious proposal.
It looks to me like Gould's proposal puts the imprimatur of government on the promotion of religion, which violates both the U.S. and Arizona Constitutions. A contrary argument would be that there's no financial expenditure by the government, since the fees to produce such plates come from the individuals rather than the government. But by allowing the expression of a particular religious sentiment (supporting monotheistic religions) and not other religious sentiments (including disbelief in any religion), it will clearly favor one set of viewpoints on religion over others.
UPDATE (May 1, 2008): Correction, the Florida license plate under consideration was one which said "I believe" with a picture of a cross. The Florida legislature looks set to allow the legislation to die without passage.