Well, that's the $64,000 question.

I spoke with a CBP officer about this issue. While he didn't directly answer the question, he did say that when they search a laptop, they don't read what appears to be "correspondence" unless it appears to be "material evidence."

He also implied that refusing to provide the password wouldn't be a problem from their perspective, as they have computer techs who can examine temporarily-seized computers. While I'm sure these folks can easily defeat the basic Windows password protection, I have heard stories of CBP trying to search a computer and giving up because the machine is running Linux, Mac OS X, or some other non-Microsoft OS.

Also, one could encrypt the files, which would offer much more protection than a simple password-required login system. With strong enough encryption, it would likely be impossible for CBP's computer techs to read the files.

I don't know how the Canadian Customs folks do things, but I get the impression that they work together with CBP and follow many of the same procedures. For example, sometimes the officer will change the questions during a routine border interview to trip up anyone prepared with a cover story. One day I entered Canada and returned to the U.S. the same day. Both the Canadian and U.S. officer changed the standard "citizenship?" question to, "Where's home"?