Retrieve post thumbnail as array

The solution is to use wp_get_attachment_image_src. As per the Codex:

$attachment_id = 8;
$size="full";

// returns an array
$image_attributes = wp_get_attachment_image_src( $attachment_id, $size ); 

Although the return is said to be:

[0] => url
[1] => width
[2] => height

There is a fourth element that indicates if it is the full image (false) or an intermediate image (true):

[3] => is_intermediate