seems to vary. I"ve played in casinos where you"re allowed to show a card whilst your opponent is making a decision, but most places seem to disallow this.... in fact, many places nowadays say that if you show your hand to anyone, even someone not playing at the table, your hand is declared dead.
Also, I believe you"re allowed to intimate the content of your hand verbally, so long as you don"t spell out your specific holding... so I guess that by that ruling it would be ok to lie about your hand, but not to tell the truth