How to get the ID of a custom attribute when having the custom attribute text value?

Question

I want to update a custom attribute and I can do that like this:

$product = Mage::getModel('catalog/product')
    ->loadByAttribute('sku',"mysku");

$product->addAttributeUpdate('manufacturer', 281);
echo $product->getAttributeText('manufacturer');

In addAttributeUpdate() instead of passing 281 which is the ID I only have an attribute text so I should get the ID of that attribute text and insert it in the place of 281

How can I do that?

$attrtext = 'myatttext';
$product->addAttributeUpdate('manufacturer', $attrtext->getId() /* ???????? */);