[jQuery] Striping Sometimes Doesn't Work

The following code snippet:
$('OL.codeSample LI:even').addClass('even');
is working on one of my sites to zebra striping to list items but is
not working on another one even though both have jQuery 1.2.6 in
place. The site where it's not working is here:
http://developer.cmzmedia.com/?p=115
Any ideas?