Put your passwords in a password safe like Keepass, and keep that somewhere in the cloud, PGP-encrypted as an extra layer of security, with a private key that is encrypted as well. Create a special private key for this purpose alone. Download that file when you entered the US and decrypt it.

When exiting, you might wipe the disk if you want to be sure they won't search it when you leave.

Or don't go to USA. Other countries need to develop their conferences and ecosystems.

If your phone is logged in to email or Facebook, they can download the total contents and history of your accounts.

There doesn't seem to be much difference between making you unlock your phone (and thus all the services it's connected to) and just making you sign into your social/cloud accounts at a kiosk. Are they doing that yet? If not, they will soon, barring a successful and crystal-clear court ruling in the meantime. I guess it's time to start re-thinking having persistent available-anywhere remote access to any of our own data. That's going to suck.

If they have the password they can search it...