Reversing the order of items in a category rss feed?

In your functions.php file

function reverseorder($query_string)  
{
    return query_posts( $query_string . '&order=ASC' );
}
add_action( 'rss2_head', 'reverseorder' );

EDIT

GET parameters on RSS feed
cat – category ID (e.g. 23)
author – author ID (e.g. 14)
year – year the post was published (e.g. 2007)
day – day of the month the post was published (e.g. 17)
monthnum – number of the month the post was published (e.g. 6)
hour – hour of the day the post was published (e.g. 23)
minute – minute of the hour the post was published (e.g. 48)
second – second of the minute the post was published (e.g. 51)
p – the unique post ID to display (e.g. 629)
paged – specific page number as displayed on the main blog page (e.g. 3)