My parents share a Google account, synced to their android phones, making it impossible to know who I'm sending gtalk messages to and for them to determine if they've already read an email or not.

True, but those people are generally screwed from a security perspective anyways.

That becomes hilariously obvious if you look at the great lengths to which people go to hide their porn stashes on a shared computer:

http://tvtropes.org/pmwiki/pmwiki.php/TroperTales/PornStash

My favorite is the guy who uses the Purloined Letter method: he just keeps it on the desktop in a folder named "New Folder". Though some people's obfuscated unicode path name tricks are pretty nice, too.

This is true. Instead of "one machine == one person" we could adopt "one virtual machine == one person" or even "one virtual machine == one app". It's too much to ask mainstream users to understand how to do this but it's not too much to ask operating system designers to support it in a way that's deeply integrated into the OS (not something layered on top in the way we do it today) so it's transparent to the user.

This might become reality more quickly than expected. At least some mobile phone operating systems attempt to do that already.

While true, the superuser/peon distinction falls to pieces in most family use scenarios.

The article should have been called the "Minicomputer security model" BTW, because that is what we are mostly stuck with.