Push email means once it hits the server its automatically sent to your device. That means you get the email almost immediately after its sent. You dont have to wait for the iphone to auto check the server every 15min (or whatever length of time you have set) to see if there is something there.
I sincerely hope that .mac becomes push. We pay for the .mac service, honestly I think Apple owes us that much.
I don't know, i think it would be awesome to be able to do it off of Wi-Fi, it would be a great tool for businesses then even if they had iPods. It then becomes more of a complete organizational tool than say, just an iPod, or even just a day planner, or really any single organizational tool by itself. It's really the combination of all these tools into one that makes it great.
I really do hope other accounts, such as AOL, are given the push feature, because right now Yahoo is so unreliable and I don't think i will be using the exchange server anytime soon, then again i do have until JUNE.