The iPhone can't get mail through Microsoft Exchange servers unless those servers are set up to deliver messages through IMAP. This means two things: first, some organizations choose not to enable IMAP delivery due to "security concerns" meaning that users can't get their mail at all with the iPhone's native mail client; second, the "push" capabilities of Exchange servers don't work with the iPhone -- the client must manually check the server for new messages (up to once every 15 minutes) rather than immediately being notified when a new message arrives. You also don't get access to Exchange's calendaring and contact functionality, though that's a separate issue to some extent.
A new offering from a company called Synchronica, Mobile Gateway 3.0, aims to address the first concern, allowing iPhone users to receive Exchange messages -- from servers that aren't configured with IMAP -- through the iPhone's mail client. The tool does this by accessing Outlook Web Access, retrieving messages, then routing them to an account already configured on your iPhone.
We tried to give the service a whirl with our company's Outlook Web Access configuration, but couldn't get it to work. The Mobile Gateway Web site refused to accept our Exchange login credentials, no matter how we tried to format the Outlook Web Access URL. If you've had success with the service (which is free for a 60-day trial), please let us know.
It should also be noted that going to Outlook Web Access servers through the iPhone's Safari Web browser works as well as one could expect, providing full access to Exchange email, calendars and contacts. Until Apple builds full Exchange support into the iPhone's mail client, we see this route as best for gaining access to Exchange mail servers that aren't set to use IMAP.