root/front-page-cats/trunk/front_page_cats.php

<?php
/*
Plugin Name: Front Page Categories
Version: 0.3
Plugin URI: http://wordpress.org/#
Description: Select categories to display on the front page.
Author: Ryan Boren
*/ 

function fpc_where($where) {
  // Change this to the categories you want to show on the front page.
  // Example:  $cats_to_show = '1 2 3 4';
  $cats_to_show = '1';

    global $wpdb, $wp_query;

    if (! $wp_query->is_home) {
        return $where;
    }

    $cat = $cats_to_show;
    if (stristr($cat,'-')) {
        // Note: if we have a negative, we ignore all the positives. It must
        // always mean 'everything /except/ this one'. We should be able to do
        // multiple negatives but we don't :-(
        $eq = '!=';
        $andor = 'AND';
        $cat = explode('-',$cat);
        $cat = intval($cat[1]);
    } else {
        $eq = '=';
        $andor = 'OR';
    }

    $cat_array = explode(' ', $cat);
    $whichcat .= ' AND (category_id '.$eq.' '.intval($cat_array[0]);
    $whichcat .= get_category_children($cat_array[0], ' '.$andor.' category_id '.$eq.' ');
    for ($i = 1; $i < (count($cat_array)); $i = $i + 1) {
        $whichcat .= ' '.$andor.' category_id '.$eq.' '.intval($cat_array[$i]);
        $whichcat .= get_category_children($cat_array[$i], ' '.$andor.' category_id '.$eq.' ');
    }
    $whichcat .= ')';

    $where .= $whichcat;

  return $where;
}

function fpc_join($join) {
    global $wpdb, $wp_query;

    if (! $wp_query->is_home) {
        return $join;
    }

    $join .= " LEFT JOIN $wpdb->post2cat ON ($wpdb->posts.ID = $wpdb->post2cat.post_id) ";

  return $join;
}

add_filter('posts_join', 'fpc_join');
add_filter('posts_where', 'fpc_where');

?>