Apart from seemingly random upgrades because planes were full, I was unexpectedly upgraded in the past from J to F on the way back from JFK to HKG by an agent who said "that's a lot of miles you've travelled" so I assumed he could see the reason why and that it was mileage. I did get a birthday upgrade (which I read about in detail here later on!) in March this year on the same route and on the same flight my wife got the same upgrade for passing a milestone (I can't remember which one but they specifically noted this at the checkin). Interestingly at 2 million miles I got a travel blanket and at 3 million a decent travelling bag. Closing in on 4 million miles so who knows (but I've only ever met Nicolas at MPC events, he's never popped out of the woodwork to surprise me!).