Coming off the heels of my SEO Task #1, Complete Permalinks, knowledgeable commenter Vanessa Fox said:

You want exactly one URL to every post. Pages can have multiple URLs pointing them. You want to avoid [this] by making sure there’s a unique URL for a page and that any other potential URLs for that page 301 redirect.

In my case the following two URLs pointed to the exact same content:

http://thepursuitofalife.com/2008/11/16/two-great-pictures/
and
http://thepursuitofalife.com/two-great-pictures/

I had enabled permalinks in WordPress, with a custom structure of /%postname%/. For some reason though, the version of the URL with year/month/day/postname/ redirected to /postname/

The following custom 404 Perl script fixed the problem: (whitespace removed for HTML purposes)

$qs = $_SERVER['QUERY_STRING'];
$pos = strrpos($qs, '://');
$found = 0;
$pos = strpos($qs, '/', $pos + 4);
$uri = substr($qs, $pos);
$pattern = '/(\d{4})\/(\d{2})\/(\d{2})(?P