Index: wp-includes/ms-functions.php
===================================================================
--- wp-includes/ms-functions.php	(revision 27089)
+++ wp-includes/ms-functions.php	(working copy)
@@ -1468,8 +1468,7 @@
 
 You can log in to the administrator account with the following information:
 Username: USERNAME
-Password: PASSWORD
-Log in here: BLOG_URLwp-login.php
+Set your password and log in here: PW_SET_URL
 
 We hope you enjoy your new site. Thanks!
 
@@ -1483,6 +1482,7 @@
 	$welcome_email = str_replace( 'BLOG_URL', $url, $welcome_email );
 	$welcome_email = str_replace( 'USERNAME', $user->user_login, $welcome_email );
 	$welcome_email = str_replace( 'PASSWORD', $password, $welcome_email );
+	$welcome_email = str_replace( 'PW_SET_URL', get_user_reset_password_url( $user->ID ), $welcome_email );
 
 	/**
 	 * Filter the content of the welcome email after site activation.
Index: wp-includes/user.php
===================================================================
--- wp-includes/user.php	(revision 27089)
+++ wp-includes/user.php	(working copy)
@@ -1997,3 +1997,38 @@
 
 	return $user_id;
 }
+
+/**
+ * Get the URL for the reset password screen.
+ *
+ * @param  int    $user_id
+ * @return string URL
+ */
+function get_user_reset_password_url( $user_id ) {
+	global $wpdb;
+
+	$user = get_userdata( $user_id );
+
+	// Generate something random for a password reset key.
+	$key = wp_generate_password( 20, false );
+
+	/**
+	 * Fires when a password reset key is generated.
+	 *
+	 * @since 2.5.0
+	 *
+	 * @param string $user_login The username for the user.
+	 * @param string $key        The generated password reset key.
+	 */
+	do_action( 'retrieve_password_key', $user->user_login, $key );
+
+	// Now insert the key, hashed, into the DB.
+	if ( empty( $wp_hasher ) ) {
+		require_once ABSPATH . 'wp-includes/class-phpass.php';
+		$wp_hasher = new PasswordHash( 8, true );
+	}
+	$hashed = $wp_hasher->HashPassword( $key );
+	$wpdb->update( $wpdb->users, array( 'user_activation_key' => $hashed ), array( 'user_login' => $user->user_login ) );
+
+	return network_site_url("wp-login.php?action=rp&key=$key&login=" . rawurlencode($user->user_login), 'login');
+}
\ No newline at end of file