Twitter.com has been compromised by some sort of a malicious virus. My account was compromised this morning along with millions of other users. It’s not clear whether the virus takes over your computer or not. An AP story suggests that it might. For the time being, avoid Twitter.com like the plague. If you need to access your Twitter account, use third-party software to do so (like Tweetdeck, Hootsuite, etc.).
UPDATE: News reports are now saying that Twitter.com has been fixed. Here’s the article from PC World.