If you are doing mobile device/browser detection on your web/app server, how should you be treating the iPhone? Well, first off here is what the User-Agent for the iPhone looks like:
Mozilla/5.0 (iPhone; U; CPU like Mac OS X; en) AppleWebKit/420+ (KHTML, like Gecko) Version/3.0 Mobile/1A543a Safari/419.3
For statistics purposed you might want to be detecting this as a phone or PDA, but for rendering it should be treated just as any desktop browser; one that doesn't support flash. There is no WML or xHTML-MP support, and from a mobile content provider standpoint, there is not a lot you can do with the iPhone. Just feed it regular HTML or xHTML pages.
Also note that the iPhone is not using the AT&T mobile gateway so you are not going to get any of the headers that tip you off that it's a mobile device; not even an x-up-subno. Good thing it supports cookies, although as always I recommend session tracking for all mobile devices using URL rewriting.
Right now it's obvious that the iPhone is coming from AT&T, but as other network operators launch it, it may become difficult to detect the network operator due to lack of headers. You can track the remote-host or remote-addr, but if the phone is in WiFi mode, you are going to end up with Host/IP data that doesn't map to a network operator.