How to use Ubercart product SKU as a condition in Rules?
This answer does not use Rules, but rather hard-coding inside of the cart template.

As the message should be displayed each time the user is on the /cart page, something like this might work. If page--cart.tpl.php doesn't exist, create it and add ...

$items = uc_cart_get_contents();
foreach($items as $item) {
    if($item-&gt;model == 'ABC123') {
        drupal_set_message('Item ABC123 is in the cart');
    }
}