A WordPress plugin that works up to at least 2.5.1 that does threaded comments and email notifications, and seems to work fine so far. (The templates it includes are pretty bad, though – I had to put together my own.) If this had gone up a few days later, I’d suspect that the guy timed it to coincide with LiveJournal’s most recent episode of cleverness.
(I still haven’t found anything that mimics friends-lock functionality perfectly. It would probably have to combine OpenIDs and something like this.)