Doesn't work with spellchecker and suggestions

This is a result of no form alter being done in the multisite module. Here's a copy-n-paste form alter that can be used for 6.x-2.x (note that this code is in the patch in #817702: Make compatible with apachesolr 2.x #11).

/**
 * Implementation of hook_form_[form_id]_alter().
 *
 * This adds spelling suggestions, retain filters to the search form.
 */
function apachesolr_multisitesearch_form_search_form_alter(&$form, $form_state) {
  if ($form['module']['#value'] == 'apachesolr_multisitesearch') {
    $response = apachesolr_static_response_cache(NULL, 'apachesolr_multisitesearch');
    if (variable_get('apachesolr_search_spellcheck', TRUE) && $response) {
      // Get spellchecker suggestions into an array.
      if (isset($response->spellcheck->suggestions) && $response->spellcheck->suggestions) {
        $suggestions = get_object_vars($response->spellcheck->suggestions);
        if ($suggestions) {
          // Get the original query and replace words.
          $query = apachesolr_current_query(NULL, 'apachesolr_multisitesearch');

          foreach ($suggestions as $word => $value) {
            $replacements[$word] = $value->suggestion[0];
          }
          $new_keywords = strtr($query->get_query_basic(), $replacements);

          // Show only if suggestion is different than current query.
          if ($query->get_query_basic() != $new_keywords) {
            $form['basic']['suggestion'] = array(
              '#prefix' => '<div class="spelling-suggestions">',
              '#suffix' => '</div>',
              '#type' => 'item',
              '#title' => t('Did you mean'),
              '#value' => l($new_keywords, $query->get_path($new_keywords)),
            );
          }
        }
      }
    }
  }
}

Marked duplicate of #817702: Make compatible with apachesolr 2.x

Keeping open for 1.x, actually.

Here is a patch that work with apachesolr 1.6