Here is a freeware utility that I've been using for about 5-6 years, it runs in the tray and allows you to program functions (about 20) like "always on top" and "minimize window to system tray", it works for outlook. It uses less than 5k in resources.
If you use it, after intall it, right click on it and select configure.
I am unaware of an Outlook setting that allows this option, but I currently only have 2000, you could probably search for a registry tweak though.
edit 1 - oops, forgot the linkhttp://www.thewonderfulicon.com/
One other note, I've found it is good to use Ctrl+Alt+Shift hotkeys, as if you use just control, or atl, you may get different results in applications, but just play around with it and change it if you get conflicts. Example Ctrl+Alt+Shift+M for Minimize to tray, Ctrl+Alt+Shift+T for toggle always on top
One other edit, I originally heard about this through PC World and PC Magazine recommending it, if you are worried about it's credibility, if you google you should see the write-ups.