There's no built-in way to do this. However, it may be possible to write an Apple Script to accomplish this. You'd either need to write a script to find the published .ics file and convert the items from events to "to do" items. Or you'd need to write your own script that creates a similar .ics file.
We started discussing this on another thread:
http://forums.omnigroup.com/showthread.php?t=25301