wp_next_scheduled() returns null but wp_options 'cron' shows my event

Hi,
I have the following code in a simple plugin, which is supposed to register an action when the plugin is loaded:

// run tablecache_activation on plugin registration
register_activation_hook(__FILE__, 'tablecache_activation');

// create an 'action' that will run once
add_action('cache_coursetables', 'uothelper_updatecoursetables');

function tablecache_activation() {
  // this method only registers via the plugin registration interface
  if ( !wp_next_scheduled( 'cache_coursetable' ) ) {
    wp_schedule_event(time(), 'twicedaily', 'cache_coursetables');
  }
}

When I look direction in the wp_options table of my database, the value of ‘cron’ is reported thus (added some line returns & bold to make it easier to read):

a:7:{i:1327964590;a:3:{s:16:"wp_version_check";a:1:
{s:32:"40cd750bba9870f18aada2478b24840a";a:3:{s:8:"schedule";s:10:"twicedaily";s:4:"args";a:0:{}s:8:"interval";i:43200;}}s:17:"wp_update_plugins";a:1:
{s:32:"40cd750bba9870f18aada2478b24840a";a:3:{s:8:"schedule";s:10:"twicedaily";s:4:"args";a:0:{}s:8:"interval";i:43200;}}s:16:"wp_update_themes";a:1:
{s:32:"40cd750bba9870f18aada2478b24840a";a:3:{s:8:"schedule";s:10:"twicedaily";s:4:"args";a:0:{}s:8:"interval";i:43200;}}}i:1327964604;a:1:
{s:19:"wp_scheduled_delete";a:1:{s:32:"40cd750bba9870f18aada2478b24840a";a:3:{s:8:"schedule";s:5:"daily";s:4:"args";a:0:{}s:8:"interval";i:86400;}}}i:1327986895;a:1:
{s:18:"cache_coursetables";a:1:{s:32:"40cd750bba9870f18aada2478b24840a";a:3:{s:8:"schedule";s:10:"twicedaily";s:4:"args";a:0:{}s:8:"interval";i:43200;}}}i:1328018823;a:1:
{s:24:"akismet_scheduled_delete";a:1:{s:32:"40cd750bba9870f18aada2478b24840a";a:3:{s:8:"schedule";s:5:"daily";s:4:"args";a:0:{}s:8:"interval";i:86400;}}}i:1328019676;a:1:
{s:17:"wp_db_backup_cron";a:1:{s:32:"40cd750bba9870f18aada2478b24840a";a:3:{s:8:"schedule";s:5:"daily";s:4:"args";a:0:{}s:8:"interval";i:86400;}}}i:1328029548;a:1:{s:10:"ai1ec_cron";a:1:{s:32:"40cd750bba9870f18aada2478b24840a";a:3:{s:8:"schedule";s:5:"daily";s:4:"args";a:0:{}s:8:"interval";i:86400;}}}s:7:"version";i:2;}

However, the event doesn’t seem to be running, and
wp_next_scheduled('cache_coursetables') returns the empty set. Is there something obviously wrong with my code?

Thanks much for you help!
Matt