There's a new sign-in seal on the Yahoo! login page, which is intended to make phishing attacks more unlikely.
A sign-in seal is a secret message or photo that Yahoo! will display on this computer only. Look for it every time you sign in to make sure you're on a genuine Yahoo! site. If the message, photo, or colors are different, you may have landed on a phishing site.
There might be other techniques to fight phishing, but it is certainly smart to raise awareness. And the technology behind it is clever too. Of course a normal browser cookie would go away from time to time when you (or the browser) clear your cookie cache. So Yahoo! uses so-called Flash SharedObjects which are sort of Flash cookies. They're available cross-browser, and they won't go away normally, because not many people are aware of how to clear these objects.