Players attempting to join servers in Battlefield 3 on the PC today may be experiencing disconnects, especially when connecting to ranked servers. The reason for this is because Evenbalance, the company that handles the Punkbuster anti-cheating software, is having problems with its authentication servers.
Punkbuster is the software responsible for keeping Battlefield 3 "hack-free", though its effectiveness is debatable. The software has also been used to purge hackers from previous Battlefield titles, and other online games, including Far Cry 3, Blacklight: Retribution and the Assassin's Creed series. The software exists in competition with Valve's VAC (Valve Anti Cheat) system, which is mainly utilized by Valve titles and multiplayer games on Steam.
Punkbuster servers have been the subject of DDoS (Denial of Service) attacks in the past, and the reason they're offline today may be due to an on-going attack. As of this time, Evenbalance has yet to release any statements regarding the present status of its servers.
EA Games is fully aware of the problem and is investigating. According to EA Games community manager Bastian, players should have no problem joining servers with Punkbuster set to "off", so if you're affected by the issue, simply filter out Punkbuster-enabled servers on your search list in BattleLog.
As a temporary solution, the Battlefield 3 forums moderators have suggested that players download and install a temporary fix, which disables Punkbuster from running and allows you to join ranked servers. In spite of their suggestion, the moderators do not take any responsibility for any errors the fix might cause, and urge players to wait it out or simply join unranked servers for the time being.
We'll update the situation as it develops.