hola euforico,
to show any random posts from a single category you should specify this string to the "query_post" function (instead of using "wp_query"):
// show me 10 random posts from category ID=9
<?php $my_query = new query_posts("cat=9&showposts=10&orderby=rand"); ?>
<?php while ($my_query->have_posts()) : $my_query->the_post(); $do_not_duplicate = $post->ID; ?>
<?php the_title(); ?>
<?php the_content(); ?>
<?php the_excerpt(''); ?>
<?php endwhile; ?>
// show me 12 random posts from categories ID's 1,2,3
<?php $my_query = new query_posts("cat=1,2,3&showposts=12&orderby=rand"); ?>
<?php while ($my_query->have_posts()) : $my_query->the_post(); $do_not_duplicate = $post->ID; ?>
<?php the_title(); ?>
<?php the_content(); ?>
<?php the_excerpt(''); ?>
<?php endwhile; ?>
I also recommend you to pass by Anieto2K's blog, he has interesting stuff about controlling this function.
just worked for me! so glad i've found this other post with the missing parameter.