I mentioned in a previous article that you can use cron to update your macs at scheduled times. The drawback with this is that if the machine is not on at that time the task will not run. So as an alternative I noticed that in 10.4.x there is a script that runs daily. So instead you can add your softwareupdate command in this...

sudo pico /private/etc/daily

Scroll all the way down to the bottom and put this line in...

softwareupdate -i -a

CTRL+X to exit, say yes and overwrite the file

You can test this by running...

sudo periodic daily

To see the output you can open the Console application in the Utilities folder. Press Logs and look for "daily" in the list on the left.