McAfee apparently has discovered a severe flaw in Yahoo Messenger, one that could ultimately lead to system compromise. Not particularly surprising, the flaw lies within the webcam functionality of the IM client, in which accepting a cam invite could result in code execution, assuming the person offering was intending to do such.
They state that there aren't any known instances of this being exploited nor have they seen any code that could exploit, merely that it is a possibility. This flaw is separate from what was found and patched a few months ago. It can only be exploited from accepting a cam invite, and unless you have a habit of accepting those from strangers it's nothing to worry about.
This is yet another flaw, however, in the version 8 series of the client. You can read more about discovery of the flaw at the McAfee security blog.