If they were genuine coins to start with, then technically yes they are legal tender. But you will probably have a problem getting anybody to actually accept them.
I would say plating is like any other PMD. As long as someone can tell it was an authentic coin it should at the very least be redeemable by the US government. Like Doug said, you have have trouble getting others to accept it though.
Side note: As "Coin Chat" is not a US-specific part of Coin Talk, let me add that in some countries the answer to the original question may very well be "No". Christian