Facebook makes it very easy for everyone. As a developer, there is a place in the Facebook Connect settings to put a url that they will send the user's id to in the event that the user disables their Facebook account but wants to keep their account with your site.

Our site http://www.wildlifenearyou.com/ does this - I wouldn't say we've got it completely right yet (we don't make it nearly easy enough to find the feature, and it's pretty ugly) but you can associate multiple OpenIDs with a single account, or set a password for it, or both: http://www.wildlifenearyou.com/account/associations/

We don't support Facebook Connect yet - I've been holding out waiting for them to provide a non-JavaScript dependent option.

TripAdvisor allows you to convert an account that you make when FBConnected into their own account.