Can they force people to reveal the password of an encrypted disk or a Truecrypt container? If someone refuses to give the password can they be denied entry into the country?
I think Truecrypt hidden volumes can be useful in such cases.
Only double-encrypted volumes provide plausible deniability. And even then you're not 100% safe. You can absolutely be arrested/detained/jailed/denied entry for not giving up a password.
This is another argument for using cloud storage as much as is feasible, with the files stored in the cloud themselves being encrypted by encryption software you trust (no relying on the cloud provider to do your encryption for you). They can't order you to give up a password to a file that's not on your laptop. Working this way adds some effort to your workflow, but nobody ever said security was easy.
I don't believe you can be denied entry to your home country. You might be arrested once you're admitted but I believe that at least a US citizen can't be denied entry to the USA.