Every Department of Motor Vehicles has a pretty strong stance against profane license plates. All curse words, both real and slang, get red flagged when you submit your request. I even remember a man in Florida who was upset because the DMV wouldn't renew his license plate that said "ATHEIST." Not quite sure why that word itself is so offensive. I'm sure they would have renewed "WRITER" or "PACIFIST." Anyway, I digress.
Here's the idea I just came up with. Start a business where you sell sheets of alphabet stickers that are replicas of the lettering they use in all 50 states. Conceivably, you could build the message around your license plate without arousing suspicion with the DMV. For example, it's incredibly doubtful that the license plate "SHITHEAD" would go through... However, how about one that says "HITHEA." Then, you just add the "S" on the left and the "D" on the right. Add another sticker that says "Stop Tailgating" and it becomes...
See. Now you've effectively conveyed your message and managed to subvert the community standards imposed by the state. This idea would really be an effective way of proclaiming your stance on abortion, dependence on foreign oil, basically anything. Political statements would be the way to go.
RAH PAL doesn't seem like much. Add an SA and an IN - Now you have the ability to make very terse and caustic statement regarding McCain's VP running mate.
I think this could spark the next wave in vanity plates. You just have so much more material to work with. Of course, it does involve adding stickers to your vehicle which is sometimes viewed as obnoxious and hippie-trashish, but I think if the message is strong enough, you'll be vindicated.
My friend Jess said this wouldn't work if you have a dark-colored car. You could make it work - just have box lettering with a white background. Although I agree, it wouldn't look as good. I think this idea has merit. Very difficult to market though. Maybe an arts and crafts store or ebay. Not sure how you'd go about promoting it though.