On my SciFoo California trip I eventually have had enough time to test my iPhone as a tourist device. The following tasks have been regularly done by my iPhone while walking in San Francisco, Palo Alto, Berkeley and at the Googleplex:
– extensive Google Maps usage (Google Maps is the poor man’s GPS as I’ve read somewhere), finding complicated places (Curtis Pickering, thanks for the ride) on the top of the hill during a cloudy night. There was no need to use paper maps.
– making photos with the iPhone camera (for instance the one on the right)
– sending those photos to my wife and family one by one (you cannot attach more than one photo at once)
– reading, deleting emails (why there isn’t any “add star” function?)
– answering emails (max 2 sentences during a walking tourist mode)
– listening music and podcasts
– watching videocasts on public transportation tools, like the good BART system
– scanning through my Google Reader feeds
– browsing web sites
– checking the SciFoo wiki
– showing some science related photos to other SciFoo campers, i.e. making little 1-2 slide presentations
– looking for the weather forecast
– when flying using the amazing World Clock feature
– making quick Notes
– making phone calls, building the contact list
Everything was fine except battery life, which was shortened to 5-8 hours of extensive multitasking usage with relax periods. So the iPhone is a terrific half day tourist device.
Deep, healthy lifespan extension 
One two lines allowed per email? Really? If you had any other phone released in the past couple years, you could always download the Gmail client, which allows both starring and arbitrary message lengths.
No, what I wanted to say with the “max 2 sentences” restriction is that it is a comfortable length for me when writing an email while in a tourist walking mode. No restrictions in the email client on iPhone, of course.