Theymos of Bitcoin Talk has advised that a successful social engineering attack on the ISP NFOrce enabled an attacker to gain access to the server which hosts the once popular forum. Theymos writes (archive):

The forum's ISP NFOrce managed to get tricked into giving an attacker access to the server. I think that the attacker had access for only about 12 minutes before I noticed it and had the server disconnected, so he probably wasn't able to get a complete dump of the database. However, you should act as though your password hashes, PMs, emails, etc. were compromised. The forum will probably be down for 36-60 hours for analysis and reinstall.

Updates as to the status of the forum can be read via the @bitcointalk twitter account.