> [the US government] have no legal means of making me tell them what my passphrase is?
Exactly. There was a lot of back and forth about legislation in the UK that basically considered it contempt of court (or worse: impeding an investigation) to not provide encryption keys/pass phrases. Problem was, the government didn't have to prove that you actually had (access to) the keys in question -- effectively being caught with a usb key filled with random data could get you a year in prison...
If you think about it it's entirely unreasonable to demand encryption keys (to say nothing of how you would prove/disprove that the right keys have been provided, specially with stuff like hidden encryption partitions available...).
At any rate the (technical) problem with lavabit was that lavabit could technically sabotage their product in such a was as to gain/give access to user data -- a secure system would not.
The viability of both private (data) and private meta-data messaging have been debated a few times, but basically it's currently hard to beat regular email+gpg+mixmaster routing.
And as long as we don't have secure "web" crypto, the situation won't change.