A couple of weeks ago I took a road trip up to Portland for my college reunion. I took three days to drive up the coast and two days back on I-5, twittering all the way. I had so much fun I made a custom Google map with annotations, photographs, tweets, etc. Embedded below, but you really need to see the full view to read the annotations. I've always loved road trips: my US tour in 1996 is one of the most important experiences in my life. Having an iPhone along for a road trip is entirely transformative. Maps that know where you are, directions, email, Twitter, Yelp, music, and a telephone all in this little device.
And, importantly, geocoding. There's no reason your iPhone can't annotate every tweet and photo with your exact location. A bit of extra battery life and it can keep a GPS track for you too. I made the Google map by hand in about two hours. But it'd be a great and doable product to automate that whole process, give iPhone owners an easy way to create their own travel diaries. The Google Maps presentation isn't perfect but it's very easy to produce and is a good start.