I think a pid file in /var/run is the right approach, maybe because that's what we do. You can easily have the cron print to stdout that it is dying because it sees the pid file of an earlier process. That stdout goes to the email address of the cron owner who always (well usually) pays attention to such things.

It is also problematic if your script dies without logging or otherwise informing you. It's not always possible, but you should try hard to make autononmous scripts log and generate nasty grams when things go wrong (by trapping signals, evaling things that could die, etc).

Phil