A nasty little exploit has just occurred on the official Twitter website. Putting your mouse over a tweet will automatically retweet a message to your followers. In the unfortunate case of Sarah Brown (wife of the former British Prime Minister), malicious content and pr0n was what showed up first. This security exploit has not been patched as of yet. The exploit does not affect third-party apps in any way (i.e Seesmic, Tweetdeck, Twitter Mobile site, and official Twitter apps).
Simply log off the official site for now.