A large distributed brute force attack against WordPress sites is understood to be occurring. A large botnet with more than 90,000 servers is attempting to log in by cycling through different usernames and passwords.  
Read more...  
http://www.siliconrepublic.com/strategy/item/32269-major-brute-force-attack/ 
http://thehackernews.com/2013/04/massive-brute-force-attack-targets.html