WordPress MU

In blogs created by users, in the "Reading Options
Blog Pages
Show at most:" 10 is the pre-selected dafault value.
How can I change this to a different number as default?
what file to edit please?

options-reading.php I guess... hope you'll come back to read my answer... hihihihi:)

I've laughed at this poor guy and found out that it's not as easy as editing options-reading.php. Anyone knows where to search for default value of posts displayed on front page. Which .php file? I want to set this value for all future posts so I don't have to change "Blog pages show at most" for every new blog created. Thanks.

there is a plugin which could help you:

<?php
/*
* Plugin Name: WPMU Blog Defaults
* Plugin URI: http://paidtoblog.com
* Description: Change the default options for WPMU and new blogs automagically
* Version: 1.0.2
* Author URI: MrBrian
*/

//------------------------------------------------------------------------//
//---Remove permalinks from menu------------------------------------------//
//------------------------------------------------------------------------//
/*function remove_permalinks_menu_item()
{
    global $submenu;
    if(!is_site_admin()) $submenu['options-general.php'][35] = '';
}
add_action( 'admin_menu', 'remove_permalinks_menu_item' );*/

//------------------------------------------------------------------------//
//---Change the permalink structure and default blogroll------------------//
//------------------------------------------------------------------------//

function change_newblog_defaults( $blog_id, $user_id )
{
    global $wpdb, $current_site, $wp_rewrite;
    //--change permalink structure
    switch_to_blog($blog_id);
    $permalink_structure = '/%category%/%postname%/';
    $wp_rewrite->set_permalink_structure($permalink_structure);
    $wp_rewrite->flush_rules();
    $wpdb->query( "UPDATE {$wpdb->terms} SET name = 'Articles', slug = 'articles' WHERE term_id = 1" );
    //--end

    //--change default blogroll
    $wpdb->query( "DELETE FROM {$wpdb->links} WHERE link_id = 1" );
    $wpdb->query( "DELETE FROM {$wpdb->links}WHERE link_id = 2" );
    $wpdb->query( "INSERT INTO {$wpdb->links} (link_url, link_name, link_category, link_owner, link_rss) VALUES ('http://" . $current_site->domain . $current_site->path . "', '" . $current_site->domain . "', 1356, '$user_id', 'http://" . $current_site->domain . $current_site->path . "feed/')" );
    //--end
    restore_current_blog();
}
add_action( 'wpmu_new_blog', 'change_newblog_defaults', 10, 2 );

//------------------------------------------------------------------------//
//---Modify the From and email used when sending emails from WPMU---------//
//------------------------------------------------------------------------//

function change_wp_mail_from($from_email)
{
	return $from_email; //return whatever you want as email, i just like it as default.
}
add_filter( 'wp_mail_from', 'change_wp_mail_from' );

function change_wp_mail_from_name($from_name)
{
	global $current_site;
	return $current_site->domain;
}
add_filter( 'wp_mail_from_name', 'change_wp_mail_from_name' );

//------------------------------------------------------------------------//
//---Change The Help and Forum links at very top of WPMU backend---------//
//------------------------------------------------------------------------//

function change_header_navigation($header_nav)
{
	global $current_site;
	$header_nav = ' | <a href="http://' . $current_site->domain . $current_site->path . 'faq/">Help</a>' . ' | ' . '<a href="http://' . $current_site->domain . $current_site->path . 'forum/">Forums</a>';
	return $header_nav;
}
add_filter('admin_header_navigation', 'change_header_navigation');

//------------------------------------------------------------------------//
//---Change default widgets------------------------------------------//
//------------------------------------------------------------------------//
//This activates the given widgets in sidebar-1 and -2. You need to know the internal names of the widgets.
function change_blog_widgets() {

   	add_option("sidebars_widgets",
		array("sidebar-1" => array("tag_cloud"),
			 "sidebar-2" => array("archives", "links")));

}
add_action("populate_options", "change_blog_widgets");
?>

I don't know where I downloaded it from, but you can set up all kind of defaults for your new blogs :-) if you figure out what setting you need to modify...

Maybe from here:

http://pastebin.com/f21c328aa

Thanks. I'm already using modified parts of this plugin on my MU installations. But this won't help me cause I don't know how default number of posts is defined. And also, I'm not really a programmer so sometimes I don't see the obvious. What I need is the name of the file where default number of front page posts is defined. Someone somewhere said it is -wp-admin/upgrade-schema.php but that file don't exist there. The one in wp-includes folder is of no use.

'They' discussed about it here:
http://mu.wordpress.org/forums/topic.php?id=3150

Also, found this:
http://rephrase.net/miscellany/05/perpage.phps

Hope sal wasnt waiting for that answer for 2 years. :D

Heh, well, this may not be the same thing you're asking (or things may have changed since then) but you can now go to Settings > Reading and change the "Show at most N posts per page" value (v. 2.6.2).