Once you land on a page created by one, you will be asked to download a new codec, whereupon you will actually receive a trojan or other malware instead. Full report from the source at Panda Security here.
This is NOT anything you get from the realyoutube site, or by watching a real youtube video. (or via any internal youtube system messages from there, etc.) To the best of my knowledge, malware cannot be passed through Youtube via a video or email systems. They have systems and/or software to prevent that from happening. They strip away any active coding from that content(s).
To protect yourself, I recommend using Firefox browser, with NoScript extension, and always surf with scripting disabled. Only temporarily allow js to be active while using the sites you need it for. Also, make sure your Java is disabled unless and until you need it. And of course, don't click links from strangers without first verifying where they lead to, or anything which seems to be designed to provoke or induce an instant emotional reaction.
This is all basic Internet 101 stuff.
Familiarize yourself with your about: plugins feature. Pretty simple. If you are asked to download a new plugin or codec, be very wary.
On a related note, there really is not too much to protect from the deadly Mind-Viruses which seem to be running amok at Youtube these days. Perhaps a tinfoil beanie might help some folks. LOL.