In my state, it is not only legal to pass on the right, but it is illegal to force someone to pass you on the right.
In other words, if you are in a left lane and someone is passing you on the right, then you are the one breaking the law, not the driver passing on the right. Slower Traffic Keep Right and Keep Right Except to Pass are the law, and the way it should be in every state. My only complaint is that the police don't enfore these laws the way they should.
Also, on a road or highway without a passing lane (i.e. "two lane blacktop), a driver being passed is required to slow down (if necessary) to enable the vehicle that is passing to do so safely.
edit: of course, every passing attempt has to be a legal one, i.e. when done so safely and in a legal lane of travel, not crossing a solid line, etc.