I previously had a script that notifies me of updates to Homebrew packages via Growl. However, I recently upgraded to Mac OS 10.8 Mountain Lion. And since Mountain Lion has a Notification Center built-in, I figured I'd try to use that method of notifications rather than Growl. I found terminal-notifier which seemed to be great for this purpose. So I updated my script to use terminal-notifier and published the script as a gist. Make sure that you have terminal-notifier installed first (just run gem install terminal-notifier).
Assuming the script is at ~/bin/brew-update-notifier, you can install the script to a crontab by running sudo crontab -e, then adding the line 0 12 * * * /Users/<username>/bin/brew-update-notifier to the end of the file (substituting <username> for your username, or wherever you've put the script). I've chosen to run the script every day at noon because my computer is likely to be on and connected to a network.