So, despite all the hullaballoo about shouting fire in a theater, there is no (federal) law against it to my knowledge, and to anyone who knows different, please correct me.
That said, there are no ante facto restrictions on speech within a theater, crowded or not (much to our collective chagrins), nor are our first amendment rights in any way curtailed by being in a theater, being in a crowded theater, or on conversations regarding fire, whenever and wherever, time, place and manner restrictions notwithstanding -- there are simply expressions that are not protected by the first amendment, which still does not entitle Congress to make such laws, even though they already have for other circumstances, and surely will again.
Edit: The bigger issue to my mind is that Congress has, and routinely skirts the issue by implementing through regulatory agency rule-making that has the effective force of law but without requiring such a law to be actually made -- for example, the ability of the FCC to enact decency bans against news agencies on public airwaves (even thought that is also on Constitutionally shaky on its own).