Twitter is reporting that they have detected unusual access attempts to Twitter user data and usernames, email addresses, session tokens and encrypyed/salted versions of password for approximately 250,000 users. Twitter says that as a precautionary security measure they have reset passwords and revoked session tokens for these accounts. If your account is one of them you should receive an email notifying you that you will need to create a new password. Twitter encourages all users to take this opportunity to ensure that they are following "good password hygiene", so yes, even if you haven't got an email it would be a good idea to change your password anyway.
In a blog post Twitter said,
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. For that reason we felt that it was important to publicize this attack while we still gather information, and we are helping government and federal law enforcement in their effort to find and prosecute these attackers to make the Internet safer for all users.