I asked the OpenStreetMap LinkedIn group this question but did not get any satisfying answer, so I'm trying with the thousands of Slashgeo users now :-)
"On the OSM wiki, there's quite a few iOS Apps listed, but a lot of them don't focus on contributing to OSM and many of them do not seem to be in active development. Thus my question: what's the best iOS App for contributing to OSM according to you? It's for an iPad 2 [with GPS]. Thanks!"