Scammers are delivering a malicious backdoor disguised as a new password file for your Twitter account. The bogus email reads as follows:
Subject: Reset your Twitter password
Because of the measures taken to provide safety to our clients, your password has been changed.
You can find your new password in attached document.
The Twitter Team
Please do not reply to this message; it was sent from an unmonitored email address. This message is a service email related to your use of Twitter.
Of course, open the attached file and instead of a new password you'll get infected by a backdoor trojan. Scammers frequently use this same password reset trick to target Facebook and MySpace users as well.
The best defense is common sense. If you receive an unsolicited email telling you that your password has been changed - ignore any links or attachments in that email. Instead, visit the service the way you normally would and attempt logging in. If the login works fine (which it undoubtedly will), you can be certain the password reset email is a malware scam.