It's somewhat discouraging to STILL see mentions of the "mu-plugins" folder scattered all through the installation instructions for a variety of plugins.
And in the code of custom-anti-spam.php I found this:
$cas_fontpath = "wp-content/mu-plugins/custom-anti-spam/";
It doesn't seem reasonable to call a plugin compatible with WP3+ if it has a path like this hard-coded. Does it?