I missed this case right when it came out, but I was just leafing through the West's Illinois Decisions paperbacks that come out before the hard volume is published, and I came across the blurb about People v. Harris, 2008 WL 733756.
I will quote the case summary from West in its entirety:
"Compliance, by a passenger in a lawfully stopped vehicle, with a police officer's request for identification was voluntary, notwithstanding that the passenger was not free to terminate the encounter. Thus, the request for identification did not violate the passenger's Fourth Amendment rights. In so holding, the Supreme Court found that the request for identification was facially innocuous, and that a reasonable innocent passenger, even upon realizing that the driver of the car in which he had been riding was about to be arrested, would have felt free to decline to provide his driver's license or other identification."
What would a typical police officer do if you refused to provide identification?
You will find out eventually because I guarantee that the next time that I am the passenger in a car I am going to withhold my ID. I might even print out a copy of this Supreme Court opinion for my glove box, not that the typical officer would give that the time of day.