Last year I wrote a blog post about how to make growl notifications (little pop ups driven by the application Growl) appear from emacs in windows. Since then I've become a total Mac-head. Here's an update on how you get that working from emacs on a mac...
Everything is the same except you need to install Growl for Mac (obv) and also make sure you copy the folder called Extras somewhere.
Open up a terminal and then cd to where you put the Extras directory. Find a sub-directory called growlnotify. Enter that and run 'sudo ./install.sh' which will prompt you for your admin password, and then go ahead and install growlnotify.
You can now run growlnotify from a terminal window. Try it out, if it doesn't work you won't be able to growl from emacs.
Now follow the instructions in my blog post above but ignore all the windows parts. You'll need to customize the growl command (M-x customize group, todochiku) and replace the full path I used for windows with just 'growlnotify'.
If everything is cool you can now do M-x todochiku-in and type a message, a time in minutes (zero for now), and pop, you should have a notification.