Celebrity Twitter List - a list here on BloggersBlog.com.
Celebrities on Twitter - a list of celebrity Twitter users created by Julie-Ann Amos.
CelebritiesUsingTwitter.com - a directory of celebrities using Twitter.
CelebrityTweet.com - posts tweets from celebrity twitterers as they happen.
Guardian's List of twittering celebrities.
Mashable's List of 25+ Twitterers.
Minor Celebrities - a list from the useful Twitter Fan Wiki.
Times Online - 50 most popular celebrities on Twitter.