Post thumbnails not working in my new install of WP 3.1 with Twenty Ten theme

Question

The featured image meta box is enabled under following conditions:

if ( current_theme_supports( 'post-thumbnails', $post_type ) && post_type_supports( $post_type, 'thumbnail' )
    && ( ! is_multisite() || ( ( $mu_media_buttons = get_site_option( 'mu_media_buttons', array() ) ) && ! empty( $mu_media_buttons['image'] ) ) ) )
        add_meta_box('postimagediv', __('Featured Image'), 'post_thumbnail_meta_box', $post_type, 'side', 'low');

So check that:

nothing messes up theme’s declaration of thumbnails support;
you are not using CPT with thumbnails support undeclared;
whatever that multisite-related stuff means, if you are using multisite (I do and I don’t remember having to do anything specific for featured images).