Trouble using orderby and meta_query for a Custom Post Type

Hi, I having a problem where I have a custom post type called webinars. I am trying to list all upcoming webinars, and have them ordered by the custom meta field called webinar_startDate.

Using the following query, I was able to return the webinars succesfully excluding the old webinars. However, they still come out in the order they were published, and not by webinar_startDate.

<?php $my_array =  array(
	'meta_query' => array(
			array(
				'key' => 'webinar_startDate',
				'value' => date("Y-m-d H:i:s"),
				'compare' => '>=',
				'type' => 'DATETIME'
			)
	),
	'orderby' => 'meta_value',
	'post_type' => 'webinars',
	'posts_per_page' => 20,
	'order' => 'ASC'
); ?>

I suspect that due to the change from 3.0 to 3.1, the use of orderby => meta_value is probably different, but I can’t find within the WordPress documentation what that change is.

Can anyone help? Thanks in advance.