orderby custom date field not sorting properly

I am attempting to organise upcoming events via a custom meta field.

The custom data is in US date format:

7/13/10
7/26/10
7/30/10
8/6/10

Etc, Problem is, wordpress is listing next years events first! so

1/9/11

gets ranked before the 10 items. Why is WP taking the highest value for the year, but the lowest value for the month and day??

here is my wp_query code:
WP_Query('cat=5,6&orderby=meta_value&meta_key=listdate&order=ASC&posts_per_page=-1');