After reading thru the replies, I was wondering if anyone would mention this. Why not just ban than from the index rather than branding them negatively. Seems stange to me.
Delisting a site for sending unsolicited email could be a bad thing... especially in the cases where it was a false report and people had actually subscribed to receive it but forgot or couldn't remember requesting it (signing up to any site might automatically set them to receive a newsletter unless they opt-out). There is a difference in warning people when visiting sites and preventing them from finding the site... it also gives the sites in question an opportunity to rectify the situation and comply with the CAN-SPAM act if they are not currently in compliance.
As long as they get their 'alerts' from trusted sources, then by all means, it's a good thing. I'm not so sure I'd be putting all my eggs in 'McAffee'. They tend to throw out false positives for this kind of stuff (one of the reasons I stopped using their toolbar). This could come back to bite yahoo though. I mean, if they list someone as a 'spammer' who has had no involvement with spam whatsoever, then they are responsible for such, and that could easily result in a very hefty lawsuit
This is an excellent addition to yahoo in my opinion. Potential abuse of the system does worry me a little though.