Don't know. But it's not a security bug. What it looks like Microsoft is trying to do is to keep backwards compatability. They ought to do like Apple and just totally trash anything older than 5 years, and stop all support for it.
Well interestingly enough, I just updated to WMP 11 and the problem hasn't arisen yet even after I revisted the websites with the content that would originally bring up the notice. SO i guess the answer is a simple upgrade... very strange.