Scope of set_time_limit

A recent note added to the manual errata pointed out the set_time_limit does not affect system commands that have been executed by PHP. I would guess that attempting to limit system behavior in this way is beyond the scope of set_time_limit. Any opinions or options on this issue? My feeling is that we document that set_time_limit is not a safe way to limit the resource usage of commands run via calls to exec, etc... --zak