Sorting posts by custom field

Hi,

How can I go about sorting posts in a specific category by a custom field? I’ve been reading up on these:
http://codex.wordpress.org/Template_Tags/query_posts
http://codex.wordpress.org/Function_Reference/WP_Query

And I gave this method a shot…but no go:
http://esdev.net/sort-events-by-date-in-wordpress-using-custom-fields/

I really just want something that works like this:
get_posts('numberposts=10&orderby=meta_key=event_date&cat='.$cat);

Also this page isn’t the main loop, its a secondary page, and i’m pulling posts by get_cat_ID.

Suggestions?

Thanks!

_n