Get all Variant SKUs using getCriteria
This should grab all of the variant's SKUs like you're looking for:

$criteria = craft()-&gt;elements-&gt;getCriteria('Commerce_Variant');
$allVariants = $criteria-&gt;find();
$allSkus = array();

foreach ($allVariants as $variant)
{
    $allSkus[] = $variant-&gt;sku;
}