wpquestions.com, ID = 2830

/**
 * Set the default column layout in the New Post section
 *
 * change every occurance of 'post' if you want to use it for pages or custom post types - example 'metaboxhidden_post' should be 'metaboxhidden_page' or 'metaboxhidden_custom_post_slug'
 */

// get current user info
global $current_user;
get_currentuserinfo();
$user_id = $current_user->ID;

// array of hidden metaboxes (removing a box from this array would cause it to appear)
$metaboxhidden_post = array (
    0 => 'tagsdiv-post_tag',
    1 => 'postexcerpt',
    2 => 'trackbacksdiv',
    3 => 'postcustom',
    4 => 'commentstatusdiv',
    5 => 'slugdiv',
    6 => 'authordiv',
    7 => 'formatdiv',
    8 => 'categorydiv',
    9 => 'postimagediv',
);

// metabox order and columns
$new_metabox_order = array (
    // the right-hand column
    // make it empty
    'side' => '',
    // the main column
    // set the order of boxes here, the ones in the $metaboxhidden_post array won't show up regardless of order
    'normal' => 'submitdiv,postexcerpt,trackbacksdiv,postcustom,slugdiv,authordiv,tagsdiv-post_tag,categorydiv,formatdiv,commentstatusdiv,postimagediv',
    // I'm not sure about this one
    'advanced' => '',
);

// update meta values
update_user_meta( $user_id, 'screen_layout_post', 1 ); // 1-column layout
update_user_meta( $user_id, 'metaboxhidden_post', $metaboxhidden_post ); // hidden boxes
update_user_meta( $user_id, 'meta-box-order_post', $new_metabox_order ); // order and columns


// disable the side column altogether (if necessary)
add_filter( 'screen_layout_columns', 'custom_screen_layout_columns' );

function custom_screen_layout_columns( $columns ) {

    $columns['post'] = 1;

    return $columns;

}