Check this : Get posts by meta data OR title
The first piece of code is working and allow to make a query based on meta data
You could just get rid of useless stuff (or adapt if you want to use several keys) :
$args = array(
'post_type' => 'wine'
,'meta_query' => array(
'key' => 'vintage'
,'value' => '2005'
,'compare' => '='
)
);
$query = query_posts( $args );