Some greasemonkey scripts also add todo lists to google calendar. Unfortunately, they are very sensitive to site fixes and updates: any update can break those nice scripts.
As an alternative, I just signed up to Remember the Milk. 'RtM' provides a rich ToDo list that integrates well in gmail and calendar:
- nice ajax input
- extensive features
- firefox extension to integrate the todo list into gmail: your inbox-view is enhanced with a RtM list on the left
- integration with google calendar