Bitcointalk.org, the Bitcoin Forum, is currently offline with the official explanation being a DOS attack. In the past, Bitcointalk.org has faced hacks, man-in-the-middle attacks, and DDOS. According to isitdownrightnow, a service that tells you the status of websites worldwide, bitcointalk.org has been down since at least 17:00 PT. This is corroborated by the first reports on twitter of the bitcointalk.org outage:
In the meantime, users can use Bitcointa.lk, which stores all of the Bitcointalk.org messages and has an additional list of features, as well.
Bitcointalk confirms the DoS attack: