WP Cron not firing at plugin activation

I’ve been going around and around with this issue on several different hosts.

On Dreamhost, register_activation_hook works just fine and dandy. It executes the class for backing up the files in quest perfectly upon activation, executes wp_schedule_event as expected and all-in-all does it’s thing.

On my MediaTemple server, on the other hand, it’s not so accommodating. register_activation_hook seems to do absolutely nothing it’s told to do. If I force the issue when I install the plugin, it will schedule with wp-cron. If I don’t, then it sits there like a dead fish.

At first, I assumed it was simply MT being…MT…but I’ve noticed this issue on other hosts as well. The company I work for maintains several different hosting accounts with several different providers, so we’ll just say this isn’t isolated to one provider.

Any ideas on the potential cause here? Here’s a brief version of the code – what works certain places, and what works everywhere.

Sometimes

register_activation_hook(__FILE__, 'registerBackup');
function registerBackup() {
    wp_schedule_event(time(), BACKUP_FREQUENCY, 'Backup');
    executeBackup();
}

Always

wp_schedule_event(time(), BACKUP_FREQUENCY, 'Backup');
executeBackup();

Unfortunately, the always block requires time and effort it shouldn’t, and I need to streamline. Thoughts?