I made 1MM last October. On the day of the flights (LAX-SLC, then SLC-LHR, 2nd flight would hit the 1MM) I was called as I was sitting in the lounge at LAX. They asked if i was still in the lounge and where I was sitting. They came over with a little gift bag and a note, which I thought was a nice touch.
On the LAX-SLC flight, the lead FA made a quick announcement at the end of the flight, just before we got off. Nice too, if a little embarrasing as some people actually clapped.
The flight where I actually hit the target, nothing was done.
All in all, it was more than I expected, and I appreciated the acknowledgment.