I had the same problem using a thematic theme child theme. The problem I found that was causing this was that the wpsc_all_products_on_page (in wpsc-includes/theme.functions.php) action that wpsc adds to the template_redirect hook exits at the end of running, therefore disabling all template_redirect actions that are scheduled to run after this action.
Among the actions that don't run are thematic_connect_functions() on priority 10, which is why the sidebars don't show.
I solved it by doing this (removing the action):
function stop_wp_ecommerce_from_messing_up_wordpress() {
remove_action('template_redirect', 'wpsc_all_products_on_page');
}
add_action('template_redirect', 'stop_wp_ecommerce_from_messing_up_wordpress', 1);
I'm not sure whether this messes up something else, I have yet to test it thoroughly. The exit() call seems like bad design.