I caved in to the urge and got an iPhone 3G . . . and have been amazed. It's not 'just' a phone, it's a tiny computer and a GPS. It instantly replaced five 'gadgets' I usually have with me: phone, iPod, computer, camera, and GPS. An unexpected bonus is that I have found it a perfect journaling tool.
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With the GPS and two very useful apps, I am always able to follow our route on topographical maps in the U.S., or using Google maps elsewhere. Topo Maps has a full set of USGS maps for download, and GPS MotionX turns the iPhone GPS into a fully functional track and waypoint tool and then some. I no longer need my Garmin, and furthermore it's much easier to use, download maps, and export and share data.
Finally, I am always using the built-in iPhone camera to take snapshots of things I want to sketch, like beetles or a nice sunset. And I can use All-in Notes to take quick voice memos or combined photo / note / memos and email them to my main computer.
All the apps are available at the App store; just use the Search function to find.