Not the original author, and yes, I know I'm commenting to an old post but..
Yes, the wp-cron job I see 500 errors on is POST. This triggers something. I'm using php-fpm, both PHP 5.3 & 5.4 versions and the current version of both WP & WPSC.
-J
[22/May/2013:12:30:54 -0700] "POST /wp-cron.php?doing_wp_cron=1369251054.8545761108398437500000 HTTP/1.0" 200 - "-" "WordPress/3.5.1; http://djparsons.com" 279 290
[22/May/2013:13:47:22 -0700] "POST /wp-cron.php?doing_wp_cron=1369255642.0790309906005859375000 HTTP/1.0" 200 - "-" "WordPress/3.5.1; http://djparsons.com" 279 290
[22/May/2013:14:51:10 -0700] "POST /wp-cron.php?doing_wp_cron=1369259470.4355010986328125000000 HTTP/1.0" 500 - "-" "WordPress/3.5.1; http://djparsons.com" 279 172
[22/May/2013:15:00:56 -0700] "POST /wp-cron.php?doing_wp_cron=1369260056.7815160751342773437500 HTTP/1.0" 200 17 "-" "WordPress/3.5.1; http://djparsons.com" 279 327