I was having the same problem. Yahoo! Mail stopped pushing after upgraded to iOS 5. Extremely annoying.
Had been researching for months for a solution. Tried everything I read: a clean restore, reboot, hard reboot, reset, reset network settings, tweaking the settings in Notifications, removing the account before re-adding it, changing the password, etc. Everything! Sadly, nothing was working.
Yahoo! does acknowledge that there is indeed a problem at their end.
Three days ago I came across NeuvaSync:
http://www.nuevasync.com/
It is a cloud-based, third-party mobile syncing service. I have been using it for the last three days to push my Yahoo! Mail and it works!