More like: "Give us the keys to this safe." and assuming they have a warrant, they can ask for such things.
You could have cat pictures encrypted with passphrase A and incriminating evidence (that stands up to the best forensic analysis currently available) encrypted with passphrase B and they wouldn't know which is which, if they unlocked ALL the data, or what.
Truecrypt is AMAZING and anyone holding onto stuff that might get them into trouble (esp. w/ foreign gov'ts) should use it.