I once was here to search for an IPhone OSM Editor. As I couldn’t find one, I decided to write one.
You can find it in the iTunes Store if you search for: osm editor
It’s iLOE (IPhone Little OSM Editor)
It’s V0.9, so I have to agree, that the user-interface needs improvement, especially for the unexperienced OSM user. The update with over 300 tags with decriptions and a lot of ui-improvement is already in the apple review process and will hopefully soon arrive.
But besides that. It works and the first nodes made with ILOE arrive from everywhere in the world.
I wrote detailed instructions in the OSM wiki:
http://wiki.openstreetmap.org/wiki/ILOE
Have fun, Johannes