[dmitrygr]

False. CBP yes. Police no. You do not have for follow their orders except for their safety. They'll have to drag you before a judge to force you to unlock it. And even then it is questionable is providing a password is protected constitutionally. Courts have ruled differently on this.

[bodz]

Not false. Police can ask for your password, and if you do not give it to them, you can be arrested pending a hearing with a judge. The judge can then compel you to give up the passcode (though they probably won't unless there's reasonable suspicion that you're hiding something - but again you're just banking on the judge's mood/feeling at that point), and if you don't, you can be held in prison until you do give it up.

The "questionable constitutionality" of it is irrelevant since in practice until there is a higher ruling on this, if you don't want to go to jail, you must give up your passcode.