I did get this to work when the iPhone first came out. It was simple as iPhone supports POP and IMAP accounts as I recall. You need to enable the SME server to allow external access to one of these preferably secure versions.
You can test initially by having a standard email client try to access your server.
I no longer have the iPhone so I can't confirm the settings but it was fairly obvious.
As an aside, I returned the iPhone because as an email device and phone it is very awkward compared to Blackberries and even WM6 based devices. However it is a very good iPod and very good mobile web browser. So for business use it wasn't terribly practical for me. Clearly my own subjective opinion mind you.