Just a data point (or 10).

When my Garmin Zumo 550 died after getting wet (!) on this summers Euro trip on my 09 GSA, I used my iPhone 4 with Navigon app and Europe maps loaded until I could get a new GPS days later. I had the iPhone mounted in a RAM ball mount on handlebars and ran power to it from my tank bag electrics. It was in a non-protective mount, so it went in the mapcase when rain threatened. I received bluetooth voice guidance to my helmet headset (in addition to all the iphone stuff like phone calls and music,etc). As a side benefit, I had live weather radar maps running some of the time to avoid storms. I navigated safely from Aix-en-Provence to Andorra (Spain) where I found a deal on a Zumo 660 to replace the 550.

As to navigation accuracy I would say it performed as well as the Zumo, both had map data/routing frustrations occasionally, but overall both work. BTW, Navigon is now owned by Garmin.

I've had the 550 refurb'd, and cannot decide whether to sell the whole kit, or keep it as a spare, or leave it on the GSA that I leave at Stefan Knopfs place in Heidelberg.

All in all, a great backup system to the Zumo which has now needed two refurbishments at $150US each because of water induced electrical issues. Dedicated units are not all they are cracked up to be, and while the iPhone is not a foolproof solution for full time navigation, it sure beats the hell out of going back to paper or being lost.

I'll certainly bring the iPhone 4 back to Europe for my next ride installment, especially now that I have had it long enough to unlock from the AT&T contract and use it with Euro SIMS I can buy as I travel for more reasonable calling while traveling.