I looked for days for a full-featured way to create iCal To Dos from messages in Mail. I tried many of the available solutions, but finally found Event Maker. This is it -- it's simple, fast, and efficient. Select a Mail message, activate the script via the Scripts menu, and Event Maker opens with all the relevant info needed to create a complete To Do (or Event or All Day Event).
[robg adds: I tried this, and it works quite well. If you're already using MailTags, you've got this functionality, but only as part of a larger Mail plug-in. Event Maker runs as a standalone application, and it can also be used to create new iCal entries directly.]
Mac OS X Hints
http://hints.macworld.com/article.php?story=2006112016493780