As for intermediate milestones, I think all the progress that has been made in AI (and neuroscience!) for the last half-century should count. We now know a whole lot more about AI and brains than we used to.

EDIT: To be more specific, I think such feats as beating humans at chess and Jeopardy! or being able to passably translate texts and drive cars are significant.

Future milestones might include: * beating the Go world champion * significantly better machine translations * much more efficient theorem provers * strong general game players

I could see all of those happening without being able to replicate all human-generated mathematical proofs. But it would still seem like significant progress has been made.