see: How To Modify New Sub Blog Immediately When Super Administrator Creates It?
The code I wanted to post I unfortunately could not post beacause "body is limited to 30000 characters; you entered 62367" so ... the following filters are handy (from WordPressMU plugin for site admin to set defaults for new blogs, Deanna Schneider) :
function set_blog_defaults($blog_id, $user_id)
global $wp_rewrite, $wpdb, $current_site;
// do stuff
// When a new blog is created, set the options
add_action('wpmu_new_blog', array(&$cets_wpmubd, 'set_blog_defaults'), 100, 2);