IBM has something of an iPhone obsession. If you search the company's site for "iPhone," a surprising wealth of results is returned. The first manifestation of that interest is, iNotes a soon-to-be released app that brings Lotus Notes to the iPhone.
IBM has been talking about iNotes for a number of months, but only recently released teaser photos of the up and coming iPhone native App. The app will bring Lotus Domino message server services to the iPhone via a web App client. It will support Domino email, calendar and contacts.
According it IBM's website the App will be built around IBM Lotus Domino Web Access infrastructure giving users access to Domino data using the iPhones rich user interface.
You can view some tentative screenshots of the App here. The site notes that the interface is subject to change, probably due to IBM's check of the interface against Apple's Human Interface Guidelines for the iPhone.
IBM has also posted documents about iPhone pp development using Ruby, Eclipse, and Netcon
This development, along with other options in the Enterprise software space like Sybase, MS Exchange via Activesync, etc. will aid in the iPhone's enterprise-level adoption.