Not that this takes anything away from MapBox's achievement, and OSM were foresighted in choosing a sanely structured data model that allows things like this.
From the product page[http://ideditor.com/]:
This has got to be the coolest non-graphing use of d3.js I have seen so far.
Anyone who hasn't should view the demo here: http://geowiki.com/iD/
I'm sure they'll add a lot of improvements eventually, but if you're looking for a more feature complete, more polished OSM editor in the interim, take a look at the ArcGIS OSM editor: http://www.esri.com/software/arcgis/extensions/openstreetmap