Tip: wp_cron() runs during init

Pro tip: wp_cron() runs during the init action at the default priority, i.e. 10. If you’d like things to be available during your cron tasks, make sure you initialize them earlier: the init action at priority 9 and less, or other actions that run before init — beware of these, since other things might not be available at that time.

Also, Core Control is a good plugin to test and debug cron tasks.