Remember the Milk (RTM) is a way to integrate a task list into your Google calendar. You subscribe to a RTM public iCalendar and the tasks will be displayed on your google calendar on the days they are due. http://www.rememberthemilk.com/
You can directly subscribe to the WebDAV calendar with Sunbird and have the tasks appear in Sunbird's task list. However, you cannot edit your tasks while offline. It would be awesome if GCalDaemon could support editing tasks offline.