I would say one of those ruberized cases -- I have an incase one for my iphone. Doug2
[ davidkearns.com - iPhone 3G ]
I must have spent a half hour touching and testing and probing and weighing all of the choices at the Apple Store at Westfield Shopping Metropolis Montgomery before settling on exactly what Doug has, though mine is black and his is yellow (they were out of yellow). I like how the Incase case doesn't cover up any part of the front, it just comes around enough to keep the device in place. The grippy back feels much better to me than the shiny silver back and as a bonus keeps the device firmly on my dashboard when driving around (which will be important when my iTrip Auto arrives).
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Disclaimer The opinions expressed herein are my own personal opinions.