Here is a way to attach files to ToDo items and Events in iCal. It works, it's handy, but it's kind of a pain in the neck to use. When you create an event or to-do, put a URL in the URL field that points to the local file you want to attach. The url has to be in this format:
file:///Users/nero/Desktop/map%20to%20Joe's.pdf
When you click on the bold "URL" label, you will get a menu that says "Go to Location." Click on that, and the file will open. This is very handy for lots of reasons.
Mac OS X Hints
http://hints.macworld.com/article.php?story=200407071753411