We've noticed that Twitter has been down for the past few hours, but now TechCrunch is reporting that the site has been hacked by a group calling themselves the "Iranian Cyber Army". There's even a message from the group. Updated.
Iranian Cyber Army
THIS SITE HAS BEEN HACKED BY IRANIAN CYBER ARMY
U.S.A. Think They Controlling And Managing Internet By Their Access, But THey Don't, We Control And Manage Internet By Our Power, So Do Not Try To Stimulation Iranian Peoples To….
NOW WHICH COUNTRY IN EMBARGO LIST? IRAN? USA?
WE PUSH THEM IN EMBARGO LIST ;)
The original note and the pictured defacement of the site are no longer online, but Twitter, status.twitter.com and other parts of the service remain down as well.
Since we don't really know exactly what's going on here, we have the same recommendation as TechCrunch: Change any passwords which are identical to your Twitter one as a precaution. There's currently no way to know if there was a security breach beyond the defacement and some extra security measures can't hurt. [TechCrunch]
Update: TechCrunch may have been too quick to flip the "Major Panic!" switch as other places are reporting that this hack was simply a DNS hijack. [Geekzone - Thanks, Randall F!]