This will run a small program that sits in your system tray that will poll for new Outlook messages, and then send a growl notification when you get one.

Currently only works with Outlook 2007 SP1 and up, and only on computers on the domain. The upside is that you dont need to supply a password.

If you need something with more bells and whistles, take a look at:
http://www.owatray.com/
http://www.owanotify.com/

Both of those didn't do what I needed, and after contacting both authors, still couldn't resolve issues so I wrote this.