I think a 'reasonable' test would be applied. If I send you a notice of something happening in a month, then it is reasonable that you can act in time, unless you are out of the country or have some other reasonable excuse. If I send you a notice of something that happens in five minutes, then I am clearly being unreasonable. Let us not forget that in addition to logic rules of 'If X then Y' there are also real people who determine when someone has violated an agreement/responsibility.
Email is different from physical home security and to compare the two I think is a bit of a red haring. That DEA case referenced where Hushmail hacked their user to get the password to decrypt their private key and stored messages shows a fundamental weakness in their system's design. I would never leave my private key on someone's server, even if it's encrypted. It's just too tempting for a government agent to strong-arm the provider into doing exactly what hushmail did. Court order? Sure, they complied with the law but here we're talking about the 'ability' of a third-party to decrypt messages. I have one key, you have another. I should be able to send to you without anyone else being able to decrypt it. Luckily, there are other systems out there like GPG which people can feel safer with. There are even some companies trying to automate everything like TrulyMail so non-technical users can also get things up and running. Is there a perfect solution? Not yet, but I see things getting better with time.
If your point is what about other encryption services (including GPG, PGP, TrulyMail, etc.) then you are right. There is nothing someone can do to prevent EVERY way of keeping messages private. The fact that RIM is talking about (or has already) given the keys to the kingdom to some governments clearly shows that they *can* knock off some tools, but not all of them. If your point is only for GPG than I agree with JSlope that it is way too difficult for the non-technical user to configure. Luckily, there are other tools which are much easier to setup and use.