<p>I'm specifically thinking about the BugMeNot service, which provides user name and password combos to a good number of sites. Now, I realize that pay-for-content sites might be worried about this (and I would suspect that most watch for shared accounts), but how about other sites? Should administrators be on the lookout for these accounts? Should web developers do anything differently to take them into account (and perhaps prevent their use)?</p>

Not make registration mandatory to read something? i.e. Ask people to register when you are providing some functionality for them that 'saves' some settings, data, etc. I would imagine site like stackoverflow gets less fake registrations (reading questions doesn't require an account) than say New York Times, where you need to have an account to read articles.
If that is not upto your control, you may consider removing dormant accounts. i.e. Removing accounts after a certain amount of inactivity.