I found a solution that effectively replaces Outlook's Calendar module with GCal:
1) Right-click on your calendar in Outlook and select Properties
2) On the home page tab, enter http://www.google.com/calendar
3) Check "Show Home Page by Default"
4) In Offline Web Page Settings, de-select the downloading web page option.
You can add a Calendar button to the toolbar within Outlook or press ctrl-2, and Google Calendar will pop up! This works for me in Outlook 2003.
Alternatively, you can assign a hyperlink to any toolbar button in Outlook so a URL will open in a new browser window.
