I did go for a "Genius Bar" appointment, and while there were a couple of other issues that they managed to 'fix', they didn't really want to deal with this mail situation. So, I have spent HOURS researching this because, I am ashamed to say, I have become completely dependent onmy email and my internet, so I needed to fix this.
While I don't have a permanent fix, here's what I know:
Apple tells me it's an ATT issue;
ATT tells me it's an Apple issue;
Yahoo tells me it's an ATT and Apple issue;
you get the picture...
I believe there is a software incompatibility between the most recent Apple upgrade in Safari and the combination ATT/Yahoo mail system, because it all began with the most recent Apple upgrade (APPLE ARE YOU LISTENING???)
Meanwhile, Yahoo's help page went down a few days ago, and when it came back up, acknowleged a problem, said its engineers are working on it, and provided a link to temporarily help us. If I log in to the link each time I begin a new sit-down with my Macbook Pro (after being away for a few hours) and then go into my mail and make sure it's online, I am able to access my mail normally. So, I now automatically log into this link and mail seems to work (they must have some clue about all this or they wouldn't be able to provide this link). SO here's the page in Yahoo help with the magic link. I hope this helps you guys!
http://help.yahoo.com/kb/index?page=content&y=PROD_MAIL_ML&locale=en_US&id=SLN52 42&impressions=true