It's a kind of grey area as far a I can tell. I don't think there are any current specific laws or regulations against it in the US but there was a court case concerning it in the 70s where a cable provider refuse to broadcast something claiming there was subliminal messaging in the tape. The FCC and advertising groups discourage it but I think that is limited to text and sounds in an advertisement being subliminal. It is generally considered a no-no even if not specifically illegal and research indicates it is not effective enough to bother doing anyway.