At the moment, it is not possible to set apache_solr as the default search.

How to activate Apache Solr as the default Search?

Any easy way to activate apachesolr on all search forms as the default search, is to download and install the module I created for this reason:

Download Apache Solr Default Search Module

I also use this module on my page. 

Code:

<?php
// $Id$

/**
 * @file
 * ApacheSolr as the default search engine by ayalon GmbH (www.ayalon.ch)
 *
 */

/**
* Implementation of hook_form_alter().
*/
function apachesolr_default_form_alter(&$form, $form_state, $form_id) {
  $search_forms = array('search_block_form', 'search_theme_form');
  if (in_array($form_id, $search_forms)) {
    $form['#submit'] = array('ayalon_search_submit');
  }
}

/**
* Custom form submit handler for the search block.
*/
function apachesolr_default_search_submit($form, &$form_state) {
  $form_id = $form['form_id']['#value'];
  $form_state['redirect'] = "search/apachesolr_search/". trim($form_state['values'][$form_id]);
} 

Download Apache Solr Default Search Module