This reminds me - is there an easy way for me to know (other than local knowledge) where the unmapped roads are in the UK. IE where I should go to next?

I guess fairly logically, I personally find WMS imagery useful for outlining the major features. In particular some things like wooded and land use areas which are not necessarily easy to walk round with a GPS. The GPS adds details that can't be garnered from imagery (and I would argue its these details that make apps and tools that are based on OSM data so much more useful day to day) and it also in many cases is much more up to date than much of the imagery.

As far as I can tell it is not a fork. There are links from the Mapquest page to 'Edit OSM Data' so it appears that it will all be contributed into the OSM pot.