It appears as though there's a mass exploitation of a wide variety of web sites injecting malicious code to gain access to visitors' computers. The exploits, once loaded, appear to sniff for passwords. More information here at SANS and the original advisory here, from US CERT.
You are strongly advised to verify that your OS has the most recent patches installed for it, particularly if you're running Windows with MSIE. If you're using Firefox, ensure that you're using the latest version, and consider using the NoScript extension to control what sites you wish to allow JavaScript on. You may also wish to disable all scripting for the next week if you're paranoid. This exploit has the potential to be exceedingly widespread, so be aware of this while you're browsing. Many of the infected sites are government, commercial, and organizational sites.
I'll keep this post stickied until the threat appears to have simmered down. I'll also be copying my request scrubber from the DESTRealm.Org forums once I'm certain that it is working, so if service is interrupted or you're unable to access certain features of the forum, just make a post here or in a separate thread, and I'll see what it's doing.
Edit: Letting this advisory decay since its seems to have passed without much incident.