There are a number of Apps for the iPhone that are generating great interest from mariners this year. (disclosure I work for Navionics). Feed back we have gotten is that at worst the Navionics App for Iphone makes a great back up, planning and route sharing tool. Navionics has spent 25 years optimizing vector charts for small screens so the iPhone match is a natural. Some actually consider it a bona fide hand held gps. The Iphone screen is great in sunlight,, the maps reside in the onboard memory. This fact along with the active gps reciever in the Iphone 3g models means that a cell signal is not necessary.
Caveats are that a good water-resistant or waterproof case is needed. I use an Otterbox. Also power management is tricky, (read short once the gps is turned on). Other companies to see are XTraverse by Fugawi and INav-X depending on the functionality you want.