Tweets on Tonight Show are fake

Just one day after the airing of Conan O'Brien's show featuring William Shatner reading offensive tweets from Levi Johnston's Twitter, O'Brien announced they had in fact been the tweets of an impostor.

On the Thursday, Nov. 5 show, Shatner appeared again, reading real tweets from Johnston.

Do you think the responsibility to correct situations like this rests in Twitter's hands?  Should Twitter apologize to Johnston and his attorney?  Should Twitter take measures to prevent things like this from happening again?