I have a Yahoo webmail address that is visible on my website under an 'e-mail webmaster' link. Starting from Sunday the Spam attack rate on this e-mail address reached truly monstrous proportions. Today it received 780 Spam messages that have shut down the webmail address completely - I can't even get on to empty the Bulkmail folder and clear the storage. I've also noted two occasions this week when I couldn't get onto the Yahoo mail site at all and received a message that the 'service is not available right now, try again later.'
It would appear that Yahoo's mail service is under some form of Denial of Service Attack - given the time frame of two days that you mention, I wonder if this latest version of the Netsky virus has something to do with it?