quey posts from different categories with taxonomy

As Bloch commented, 3.1 provides a way to easily accomplish this. Here’s a terrific article describing exactly how to execute advanced taxonomy queries. Here’s the plugin to run the 3.1 beta.

Here is some slightly modified code from an example in the advanced taxonomy queries article:

$adcatquery = wp_parse_args( $query_string );

$adcatquery['tax_query'] = array(
    array(
        'taxonomy' => 'ad-category', //custom taxonomy name
        'terms' => array( 'cars', 'trucks', 'whatever' ), //custom taxonomy terms
        'field' => 'slug', //refers to $query_string, may use ID
        'operator' => 'IN' //NOT IN excludes terms
    ),
    array(
        'taxonomy' => 'model', //custom taxonomy name
        'terms' => array( 'corolla', '4runner' ), //custom taxonomy terms
        'field' => 'slug', //refers to $query_string, may use ID
        'operator' => 'IN' //NOT IN excludes terms
    )
);

query_posts( $adcatquery );

This should give you a decent idea (haven’t tested it though)… good luck!