One thing I wonder, there have been a slew of these articles which probably push (at least somewhat) people to use online services to store their life. Will customs agents ask for passwords to your gmail, facebook, etc? If they aren't doing that now, how long before they do?
They start asking for your gmail password? The criminals will just stop using gmail.
The fact that you can just send any incriminating data over the internet instead of carrying it into the country on a laptop/HDD/etc. shows how pointless these searches are. Did I say pointless? Violating.
So basically the only people who get screwed by this are the people who haven't actually done anything wrong but forgot to delete any sensitive/embarrassing content from their drives. The criminals will just use a normal internet connection. Just as with everything else these days: violate the privacy of innocent people as much as possible so they won't notice that they're incapable or too lazy to do any real law enforcement.
Has anyone ever actually had this happen, or is this just plain ol' Guardian-style anti-American paranoia? I've carried computers through US customs as a noncitizen many times, and they've never shown any interest in searching 'em. Of course they have the power to, in theory, just like the customs of just about any other country you might enter. But are they actually exercising this power routinely?