You can be forced to identify yourself during a Terry stop.
5-4, with the usual suspects voting where they usually do; Justice Kennedy's opinion.
If the police have reasonable suspicion of criminal activity, they have long been allowed to stop you briefly in order to investigate further; this is known as a Terry stop. Using a case arising out of Nevada, the court now rules, in further erosion of our Fourth Amendment rights against government intrusion, that you can be thrown in jail for refusing to tell the police your name, so long as they have "reasonable suspicion" to be talking to you in the first place, even if they don't otherwise have probable cause for arresting you for anything.
Bah. I'll grant that it's a marginal question, but, dammit, that ought to mean that you rule in favor of the less powerful side. Getting thrown in jail is a very big deal for an individual; not knowing somebody's name is a very small deal for the government, repeated many times. The size of the individual's interest ought to overbalance the repetition of the government's small interest.