REPLACE:
$xmlstr = file_get_contents($feed_url);
------------
WITH:
$ch = curl_init();
$timeout = 5; // set to zero for no timeout
curl_setopt ($ch, CURLOPT_URL, $feed_url);
curl_setopt ($ch, CURLOPT_RETURNTRANSFER, 1);
curl_setopt ($ch, CURLOPT_CONNECTTIMEOUT, $timeout);
$xmlstr = curl_exec($ch);
curl_close($ch);
------------
Justin, if you see this, I tried with 3 different browsers to comment on the feed parser post (and the post about your spam filter) and was unable to do so.
Good work on the plugin though!
http://wordpress.org/extend/plugins/google-calendar-feed-parser/