Just some information, on Windows:
- process ids are re-used.
- parent process id is set at process creation time and never updated.
(Windows Internal 4th Ed. by Russinovich and Solomon).

Thus, I would say that a long running process can not rely on the value 
of its parent process id.  The parent may not be running, or the process 
id may have been re-used and the process with that id may not be in fact 
the actual parent.  Having said that, I don't know the algorithm that 
Windows uses for process ids, so collisions may be rare (or not).

-Do these same issues exist on Unix systems?