Mikel Maron, the champion of GeoRSS and contributor to the Open Street Map project, can now view OSM tiles on his iPhone. He has replaced Google as the data source on the iPhone’s map application. He’s got the details of how he did this on his site.
It’s definitely a handy hack to know if you have an iPhone and need your own custom maps with you. Of course these would be more useful if the iPhone had a GPS.
PS – The Navizon’s shareware GPS for the iPhone is OK, but too expensive. It is hit-or-miss for finding my location (great in SF) and doesn’t offer any additional functionality like saving traces or waypoints. $25 is too steep for an application that I may not be able to use with the next update of the iPhone’s firmware.