If you’re using PHP 5.* then it’s also possible to use simplexml to pull the src-attributes value or any other attribute value out of the get_post_thumbnail function. I worked this out:
$domsxe = simplexml_load_string(get_the_post_thumbnail());
$thumbnailsrc = $domsxe->attributes()->src;
You can then echo or whatever you wish do do with $domsxe that now stores the src value as a string.
