Frustrated by the lack of week number support in iCal, I started searching for a solution. I stumbled upon a PHP script that generates a calendar with week numbers for Google Calendar. I changed this PHP script to make it output an iCal week numbers calender instead. Here's the modified version.
Usage:
Copy the PHP script, save it as generate_weeks.php or whatever, and run it. Save the resulting calendar with an .ics extension. Create a new calendar in iCal, called something like week numbers, select it, go to File -> Import, and import the generated .ics file.
For people that don't have PHP at their disposal, you can download the 2007 and 2008 calendars. You now have a calendar that displays the week numbers above the weeks.
[robg adds: I also created an archive with the PHP script and the two calendars. To run the PHP script, just type php scriptname.php in Terminal.]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20070104091628809