Updated query to reflect added ContentTypeIdentifier by @crevillo in 0cb178324a63767664dc0aed0f2ddd1ace7d1e1d in January 2013

new_way.php

<?php
$locationService = $this->getRepository()->getLocationService();
$searchService = $this->getRepository()->getSearchService();

$query = new \eZ\Publish\API\Repository\Values\Content\Query();
$query->criterion = new \eZ\Publish\API\Repository\Values\Content\Query\Criterion\LogicalAND(
    array(
        new \eZ\Publish\API\Repository\Values\Content\Query\Criterion\ParentLocationId(
            2
        ),
        new \eZ\Publish\API\Repository\Values\Content\Query\Criterion\ContentTypeId(
            array( 1, 20, 21 ) 
        )
    )
);
$query->sortClauses = array(
    new \eZ\Publish\API\Repository\Values\Content\Query\SortClause\LocationPriority()
);
$results = $searchService->findContent( $query );
$items = array( );
foreach ( $results->searchHits as $hit )
{
    $location = $locationService->loadLocation(
        $hit->valueObject->contentInfo->mainLocationId
    );
    $items[] = $location;
}

old_way.php

<?php
eZContentObject::subTreeByNodeID(
    array(
        'ClassFilterType' => 'include',
        'ClassFilterArray' => array( 'folder', 'page', 'contact' ),
        'SortBy' => array( array( 'priority', false ) )
    ),
    2
);