Recently many people are getting a direct message or an email notification asking them to log-into their twitter account. When you click on the twitter link given in that DM, you will be directed to the site which looks like twitter.com. However it is not the actual twitter.com, but a phishing site. Look closely at the URL because it could be a scam and if it is not twitter.com, don't sign in.
As shown in above image, http://twitter.access-logins.com/login/ is such a phishing site trying to get your twitter log-in details.
Fortunately if you are using firefox3, you will get a web forgery notification message as shown below:
You can find more details on twitter blog.