According to CM Nethaera, there's a recent outbreak of posts on the forums linking to hostile programs (keyloggers). What's a keylogger, you ask? Well, it's a piece of software that gets installed on your computer -- either because you've installed something without knowing what it was or because the program has exploited a vulnerability on your computer and installed itself without your knowledge -- that tracks everything you type and reports the information back to someone, somewhere. Often, it's used to to steal valuable information like account passwords, credit card numbers, banking information, and the like. But in the case of World of Warcraft, it's often used to steal your account name and password -- which can then be stripped for items and gold to be resold for cash.
Certainly not something any of us would like to happen to our accounts -- so it's probably a good idea to heed Blizzard's warnings. Specifically, we're warned against links to a file named tonydanza.a11net with the extension .html. So if you see something like this linked from the forums? Pay attention to Blizzard's warning page that you're visiting a non-Blizzard site and don't click it!
And if you're interested in what you can do to keep your computer secure, Blizzard has some general advice on their support site. And for more specific advice, forum poster Semoravene has provided us all with more specific details on keeping your computer safe from viruses and other harmful programs (that means keyloggers!). And if you think your account may have already been compromised, the next step is to contact Account and Billing services -- but be warned, getting your account back can sometimes be a bumpy ride.