I have written a Perl script that simply checks to see how long it’s been since a particular task has been performed, compares the time with how often that task should be performed, and if necessary, executes the task. The script is extensible to any maintenance tasks that output to a log file (and any task can be made to do so). The script also includes the option to send you an e-mail (but only if the task(s) have been executed) including full script output. Also, there is an AppleScript controller which will allow you to suspend maintenance activities for discreet periods of time, so you’re machine won’t suddenly start churning during a presentation, for example.