http://www.codeplex.com/ImapPusherService
I used to forward gmail to mail2web for free push. With this little program, I can push gmail directly to my Diamond. It is very small and has been working no problem.
Usually I don't like the idea of running a 3rd-party push service such as the blot Flexmail and Emoze. But from his blog, this author does exactly what I want - small, efficient and as simple as possible, minimum impact on battery life and data usage.