I've been using Gmail with my iPod Touch and it's done a relatively good job at pushing email to my iPod Touch. Now is it true push? I don't know. I'm not too knowledgeable when it comes to that kinda' thing. What I do know however is that fixed into a Wi-Fi connection, my iPod Touch is always beeping at me while I'm dancing around in the kitchen with my earbuds in, doing dishes, telling me that I have new emails. 🙂
I'm not too picky. The emails show up on my iPod Touch almost instantly and I can reply to them on the spot. If your a business man or a little more OCD about your email, you may want to dive a little deeper to ensure you've got true push email.