Abigail,
I am not going to pretend to be authoratitive here, but I believe your very well written synopsis is true for *nix systems but not all operating systems. I believe file locking is more integrated into the OS for Win32 systems since you can't delete a locked file there where you can in *nix.

Cheers - L~R

Update: I didn't originally notice the word most which makes everything Abigail says valid. It is important to either write portable code that will behave correctly in all environments, or make the limitations of the code to a target platform well known.