http://www.todoist.com is a good service to manage your todo-list online. You have a nice Ajax webgui and support browser-plugin and mobil apps are avalibale.
As you read on the page  there is no way to integrate iCal and you todoist with a client like Thunderbird, Apple Mail / Apple Calender, Google Calender or other program to connect to iCal-Server.
The only think witch was available is a bad hack by Yahoo Pipes and has no way back.
Now we present the version of our todoist2iCal Server. You can run this one any PHP enable Webserver. It use the open API of todoist.com.
At the moment the following function are implemented:
- show all task from all project as iCal-Todo
- make task completed
- add new task to first project
- problems with priority
- due date problems on insert
You need to add your API-Key in the source. You find this under Preference -> Account on todoist.com. Have fun to try out. The code is release to GPL 2 or later.