Outlook pops up many dialogs that offer a 'never show this again' checkbox. If you check the box and change your mind, you'll need to edit the registry if you want to see the dialog again. You can also add values to this key so you (or your users) won't be presented with the dialogs ever.

To control which dialogs are shown, browse to the following registry key (replacing the xx with your version number):
HKEY_CURRENT_USER\Software\Microsoft\Office\xx.0\Outlook\Options\General

Look for the PONT_STRING value:
REG_SZ: 31,37,8,23,

You can add and remove numbers from this list. Just make sure they are separated by a colon and the last number is followed by a colon. The order apparently does not matter - Outlook added them at the end as you tell it never show you a dialog again.

For a list of all known PONT_STRINGS, see
http://www.slipstick.com/outlook/showdialog.htm

If you discover any I'm missing, let me know.

Version numbers:
Outlook 2007 = 12
Outlook 2003 = 11
Outlook 2002 = 10
Outlook 2000 = 9