Rather than just post an OPINION, I really wanted to know for sure what the truth was about this, so I made a few calls just now (hey, I'm home sick, not much else to do ). Of course, I got bounce around a bit, but I finally ended up speaking with a lady at Kentucky's Motor Vehicle Licensing, and she gave me the facts. Kentucky does not require a tag on your boat trailer, and other states are constitutionally required to honor each other's laws. I did a Google search, and found that it's the Full Faith and Credit Clause (Article IV, Section 1) of the Constitution that requires this. However, she also confirmed that it's not uncommon for police officers to issue citations anyway. If you go to court and show them a copy of the statute, you will win, but they strongly recommend just getting the tag to avoid being hassled. And she told me about one guy who went to court to fight one of these tickets, and failed to bring with him a copy of the statute, and the judge almost gave him 30 days for contempt of court, saying "It's not my job to look up other states' laws."

She also gave me another very valuable piece of information. There is a place on your registration for your signature, although in KY you are not required by law to sign it. But they are starting to hear stories of people being cited in other states for having unsigned registrations. It's the same deal, if you go to court with the statute in hand, you win, but me, I'm going sign all my registrations immediately, just to ward of hassles.