Hot!
As an example of how this handy feature works, when photographing a ball game with a player on first base, a photographer can prefocus on second base, lock the focus into the memory, then focus on whomever is up at bat, follow them (using Focus Track) to first base, press the Memory button again, and snag a sharp photo of the first player sliding into second base. When shooting fast action this feature can prove to be priceless.
Great idea.