It's called the letter of Marque and Reprisal. Ron Paul in fact introduced such legislation at the urging of some freepers. Alas Bush and the Congress have no use for the constitution.
Article I, Section 8. Got it. Do you seriously believe that privateers or mercenaries could capture OBL? We're not talking about a small group of desert thugs; we're dealing with tens of thousands of well-armed, well-trained, experienced soldiers that had the backing of the former Afgan government. We're talking about a group of people that may have WMD and are not afraid to use them now. And you want to send in Sylvester Stallone?
My view is that such a response is woefully inadequate to the point that the government would no longer be providing for the common defense. And because it is so inadequate, I believe that the government would be abdicating one of its primary functions. In this case, letters of Marque and Reprisal are unconstitutional because they are inadequate.