"How can I export my Outlook 2007 Calendar as an iCal file?"
Outlook 2007 makes publishing an iCal file a little confusing because the option isn't part of the Import and Export options. Instead of using the Import and Export wizard in Outlook, you need to be in the Calendar view and File > Save As.
By default, the Save As will export your calendar as an iCalendar .ics file of the current day.
Clicking the More Options button allows you to save as much of your calendar as you want to export as an iCalendar.