Assuming you've checked the 'Show links to iCalendars' box in your Preferences, there is an iCalendar icon and link on the Home Screen with a link to your Tasks iCalendar'. Put that URL (with your username and password) into your iCalendar application and you're all set.

The same icon appears in your Users and Groups lists (also in Preferences), you can use those URLs to subscribe to iCalendars for your group(s) or other users in your groups.