The WP-Cron functions are not actually cron functions. Instead of a cron running and executing tasks precisely as scheduled, it waits until the frontend or admin is loaded, checks if any jobs are scheduled and then fires as needed. To execute the cron jobs, WP loads wp-cron.php, which is located in the root folder. My understanding is that when you visit the site and a cron is scheduled, a request is made to http://yourdomain.com/wp-cron.php, which initiates the scheduled cron job(s). I don’t know exactly how this works (I’ve read about it before, but cannot find the resource again), but this request is made in such a way that not all servers can handle it, causing it not to work. I wonder if your problem is that this request isn’t made with a GET request to your site.
As an alternative, I would recommend using a GET request to http://yourdomain.com/wp-cron.php. In fact, in the excellent Professional WordPress Plugin Development, they recommend the following:
A common method is using wget to load
wp-cron.php on a schedule. If your
server is Linux- based, cron will
already exist and can be scheduled
using the crontab command. If your
server is Windows-based, you can
install wget and create a scheduled
task to execute it. The wget command
would work like this: wget
http://www.example.com/wp-cron.php
When wget requests wp-cron.php
WordPress looks for all scheduled cron
jobs and executes as needed. Setting
up true cron on your server will
guarantee your cron jobs will run
perfectly on schedule without missing
a beat.
Along with this, you need to disable the default way of handling cron:
define(‘DISABLE_WP_CRON’, true);
Another discussion of this can be found here:
http://caramboo.com/2010/03/wordpress-remote-cron-scheduling/