As far as I can figure, if your phone is powered off and requires a passcode to run, the only thing they can do is confiscate your device. They cannot bar a US citizen from entry into the country - even if you have no passport or identification, they can only fine you for entering without it.
If they confiscate your device I'm not sure how long they can keep it off the top of my head, but I would operate under the assumption that you're not getting it back. There are legal remedies for this sort of warrantless seizure, but I'm guessing that there's a mechanism through which they could charge the phone itself with a crime in order to circumvent the Fourth Amendment, a'la asset forfeiture.
Right, that's my understanding as well. I'd lose the device but retain my freedom and (whatever shreds remain of) privacy, which is an acceptable tradeoff to me given the relatively low odds of this happening.
The Fourth Amendment is interpreted quite differently in border zones; the side of the Government is generally weighed significantly heavier "because terrorism".
If they confiscate your device I'm not sure how long they can keep it off the top of my head, but I would operate under the assumption that you're not getting it back. There are legal remedies for this sort of warrantless seizure, but I'm guessing that there's a mechanism through which they could charge the phone itself with a crime in order to circumvent the Fourth Amendment, a'la asset forfeiture.