Yahoo email "push" stopped working for me on my iPad (3.2) and, later, on my iPhone 3G (ios4). The fixes turned out to be different on both, but both are now working...
On my iPad, I needed to Reset Network Settings (under General, Reset). It did it's thing, rebooted, and I needed to re-enter the password for my home network. Poof... Push notices came in.
On my iPhone 3G, I deleted my yahoo account and re-entered it. Push started working right away.
Hope one of these is helpful for you.