I personally wouldn't use any single event to mark the "start" of private space exploration. It's a gradual process, not something that happens all of a sudden.
If they fail they will point at this mission as an example of a foolish pursuit that should have been left in the hands of government agencies.
Either way this is, without a doubt, the start of the private sector launching into space exploration. Nobody can point to any other private mission that launched a capsule, docked at the space station and then returned cargo to earth.
put in other terms:
$ git commit -a -m 'Here we go!'