Bitcointalk.org is ranked less than 6,000 in alexa, both US and worldwide. Depicting, there are a lot of users being affected in the hacking.
Users who are browsing the forum were greeted of an animated page with a background music, and some other displays like explosions and rockets.
Theymos, the administrator of bitcointalk has already commented of what happen:
“There’s a good chance that the attacker(s) could have executed arbitrary PHP code and therefore could have accessed the database, but I’m not sure yet how difficult this would be. I’m sending out a mass mailing to all Forum users about this.”
Fortunately, there are backups of the forum and several people are working to put it back it action. However, the forum won’t go back up until they know how the hacking was done. Theymos is even offering 50 bitcoins to the first person who can point out the specific security glitch.
Moreover, Theymos told that BitCoinTalk user passwords are hashed using sha256crypt with 7500 rounds but, it is still recommended to change passwords.
Embedded in this post is a video of the actual defacement, coinciding the birthday of 4chan.