I was considering the same thing, but the fact that it was working until the Yahoo upgrade seems suspisious. Especially since I now see that others are havng the same problem.
I'm beginning to think it IS a problem on the Yahoo end. I've since installed K9, and have not been getting notifications on that either. I've plugged in my phone so we'll see if I get anything. It seems odd that it would be anything on the Yahoo server side, though. I'd think mail notifications would be more about the phone reaching out to check, but maybe they actually push somthing to the service and that's not happening for some reason.
Sto Pro Veritate