I don't know why they took this option out of the settings within Outlook, but the registry setting for this is still there.
To minimize it to the system tray, start regedit and follow the steps below:
HKEY_CURRENT_USER \ Software \ Microsoft \ Office \ 10.0 \ Outlook \ Preferences
New Data Type: REG_DWORD
Value Name: MinToTray
Data: 1 (0 to disable MinToTray)
The number in red above is the MS-Office version number; 10 is Office 2000; 11 is Office 2003.