I found the old link (that used to be on the Help page; it is no longer there, but it still works). Here it is (click on it to log off Amazon): http://www.amazon.com/gp/help/customer/sign-out.html/
There also is (yes, there is still) an official log-off link, only it's on a different page now: it used to be on the Help page, whereas now it's on the "Your Account" page. Go there and look at the right/top field saying "you're logged in as [your name]. You should be logged in, of course; when you are, right under your name there will be a "Sign Off" link.
So the bottom line is, it is still possible to log off. Don't forget to do so after using a public computer.