I was having issues with Yahoo email not pushing. I kind of got it fixed. I've narrowed down my problem to always having the mail app running in the backround. Before iOS 5 I would always close all my apps from multitasking and my emails would still push.
After updating to iOS 5, my emails were not pushing. I tried deleting the account from my iPhone 3GS, then re-adding it. Also hard resets and toggling the push switches.
After annoying my wife asking her to send countless test emails, I realized they were only getting pushed with my mail app running in the background (not being closed in multitasking).
I would like to have Mail closed and still getting pushed because I'm kind of OCD like that, but for now this will have to do.
By the way, I don't have E-Mails in my Notification Center. That's the next thing I'm going to test to see if that has any affect.