Martin,

As far as California goes, in addition to Penal Code section 150, you may want to check out section 830.6(c), it says:
"Whenever any person is summoned to the aid of any uniformed peace officer, the summoned person is vested with the powers of a peace officer that are expressly delegated to him or her by the summoning officer or that are otherwise reasonably necessary to properly assist the officer."

On a practical level, in over 18 years of law enforcement experience covering courts, jail, patrol, riots (Remember Rodney?), and natural disasters, I have never nor have I ever heard of anyone deputizing a private citizen.

An interesting sidenote is Calif. Code of Civil Procedure section 211:
"When a court has no prospective jurors remaining available for voir dire from panels furnished by the jury commissioner...the court may direct the Sheriff to summon, serve, and immediately attach the person of a sufficient number of citizens having the qualifications of jurors, to complete the panel."

So you could be sitting in Starbucks enjoying your iced latte, and one of my partners could walk up and say, "Guess what? You're now on a Jury panel!"