I'm using the tweet button in a view and the fact that the tweet button link is contained in a div was breaking validation (the view field is written in a span element) so I changed :

$link = '<div class="tweetbutton"><a href="http://twitter.com/share" class="twitter-share-button" ' .
drupal_attributes($attributes). '>Tweet</a></div>';
return $link;

to :
$link = '<span class="tweetbutton"><a href="http://twitter.com/share" class="twitter-share-button" ' .
drupal_attributes($attributes). '>Tweet</a></span>';
return $link;

And it seems to pass.

Thanks for the module !