Interesting, did not know about notify-send, I was going to suggest xmessage(1).
A real cheap method, assuming your script is executed via cron, is to add this:
Code:
echo "There are updates available"
to your script. cron will then mail a message to your mailbox (file $MAIL).
I know xmessage(1) will fail if executed inside cron, not sure about notify-send.
John