From the Twitter blog post, Twitter Blog: Keeping our users secure:
Even if you didn't receive an e-mail from Twitter indicating your account was compromised, it is strongly suggested that you change your password anyway. Better safe than sorry.
A couple of articles of interest:
This week, we detected unusual access patterns that led to us identifying unauthorized access attempts to Twitter user data. We discovered one live attack and were able to shut it down in process moments later. However, our investigation has thus far indicated that the attackers may have had access to limited user information – usernames, email addresses, session tokens and encrypted/salted versions of passwords – for approximately 250,000 users.
As a precautionary security measure, we have reset passwords and revoked session tokens for these accounts. If your account was one of them, you will have recently received (or will shortly) an email from us at the address associated with your Twitter account notifying you that you will need to create a new password. Your old password will not work when you try to log in to Twitter.
Even if you didn't receive an e-mail from Twitter indicating your account was compromised, it is strongly suggested that you change your password anyway. Better safe than sorry.
A couple of articles of interest: