rodriserick

Se alguém estiver com o mesmo problema, consegui resolver da seguinte forma:

//Create a new filtering function that will add our where clause to the query
							 function filter_where($where = '') {
							   //posts for March 1 to March 15, 2009
							   $where .= " AND post_title LIKE '".$_GET['letra']."%'";
							   return $where;
							 }
							 // Register the filtering function
							 add_filter('posts_where', 'filter_where');

							 $int_nac = $_GET['int_nac'];
							 $estilo = $_GET['estilo'];
							 // Perform the query, the filter will be applied automatically
							//$parametros_filtro_artista = ($query_string . "&cat=".$int_nac.",".$estilo."");
							 $parametros_filtro_artista = ($query_string . "&(array ('category__and' => array (".$int_nac.",".$estilo.")");
							 //echo $parametros_filtro_artista;
							 query_posts(array('category__and' => array($parametros_filtro_artista,$int_nac,$estilo)));

						?>
                        <?php if (have_posts()): while (have_posts()) : the_post();?>

                    	<a href="<?php the_Permalink()?>">
                            <img src="<?php bloginfo('template_url'); ?>/img/roster_03.jpg" alt="Artista" title="Artista" />
                            <div class="desc"><font><?php the_title();?></font><br /><?php the_excerpt_rereloaded(10);?></div>
                        </a>

						<?php endwhile; ?>
						<?php endif; ?>