Alternatively, one could make it opt-out rather than opt-in. Slashdot does this for your session - you can mark a session as being at a "public terminal", ensuring that you're logged out when your browser session ends.
Slashdot, news.yc, reddit, fark or digg aren't exactly the most important of my accounts. I sure wouldn't want my bank to leave me logged in by default, but I find the rest of my accounts to be near worthless.