I seriously doubt Yahoo was affected by this; the problem we discussed in the other thread was something that would only affect Windows machines; it is scary that the same virus/worm is able to spread via either Outlook, MSIE or IIS; it's like Sircam and Code Red had an evil child
But Yahoo runs FreeBSD (last I heard), and I very seriously doubt any Yahoo server runs Windows.
I am sure that either the original post is mistaken, or it was via Yahoo Mail (most likely the case). I'm sure if you looked at the disclaimer you agreed to when signing up for a Yahoo mail account, you'll notice that they take no responsibility for what may be sent to you.
If you were going to go after someone, it should be the vendor of the offending web server (IIS), web browser (IE) or mail client (Outlook). But of course these are all Microsoft, and of course their EULA has a similar disclaimer. They do not guarantee anything, even that it will function, much less protect you from any virus/worm...