Edit: I wonder if the bad guys were able to access private Direct Messages, too. There's always talk about resetting passwords, but nobody ever mentions all the other confidential data that might be pastebin'ed later.
This attack was not the work of amateurs, and we
do not believe it was an isolated incident. The
attackers were extremely sophisticated, and we
believe other companies and organizations have
also been recently similarly attacked.
There are moderate numbers of people who use Twitter for online activism. That would point more to Syria or Iran or another Middle Eastern country, since Twitter is more popular there than in China or the Chinese language.
Pretty clearly this was the work of governments e.g. China, Iran who are trying to find out more about political dissidents or the sources of leaks. They are the only ones who would use lucrative exploits against Twitter, NYT, WSJ etc.
Pretty interesting if you ask me, is it likely the use would sick the best hackers at the NSA if top secret information was being leaked on the bizzaro world's Chinese Twitter? It almost sounds like the prologue to the worlds first CyberWar.