There are a number of iPhone apps - such as OffMaps - which let you use OpenStreetMap data, and view the maps, on your iPhone and iPod touch, but there hadn't been any apps which let you edit the data, until now.
If you are a member of the OpenStreetMap community, you've probably been out and about at some point and looked at something and thought 'hmm, I wonder if that [insert interesting thing here] is already on the map?'. Well, now if you have an iPhone you can quickly check whether it is or not, and either add it or update its details directly in the OSM database.
The first iPhone editor to launch on Apple's App Store is Little OpenStreetMap Editor (iLOE), an app that allows you to add or edit OSM nodes wherever you are. While its user interface isn't very friendly at the moment, hopefully it will improve in the future and make it easier for users to edit the data around them. In the mean time, it does do the job, so if you're desperate to edit OSM on the move, give it a try.
Cloudmade are also working on an app - the Mapzen POI Collector - which aims to give a quick, user friendly way of editing OSM data. The app tries to make it as simple as possible to contribute to the project from your iPhone, whether you were an OpenStreetMap contributor before or not. The only thing you need to do is sign up for an account at openstreetmap.org, and the app will let you add all sorts of information without having to know anything about the tagging scheme used in the project.
Currently awaiting moderation to get into the App Store, the app should be available very soon, and is definitely worth trying out when it's available. If you're interested, you can sign up to be notified when it's out. I've tried out some pre-release versions which have been shaping up very well, as these screenshots in their tutorials show.
A number of apps are available on other mobile platforms which will let you view OpenStreetMap data, and a few of them also allow editing, including the Java-based TrackMyJourney app which I recently tried out on my Nokia N95 phone.
Update (26 November): Mapzen POI Collector is now available for the iPhone from the App Store