It should be pretty easy to write a script to clear out /tmp -- but I run 10 or so SME boxes and none of them fill the /tmp folder every couple of weeks.
Of course, my SME seems to have a job scheduled (/etc/cron.daily/tmpwatch) that looks like it clears out /tmp and some other folders...
the files in /etc/cron.daily are run daily as scheduled in /etc/crontab...
Perhaps something has happened to your default /etc/crontab or the contents of /etc/cron.daily?