ENGLISH ONLY -
This is a new 'Add to Cart' button rather than 'In Cart'.
I have experienced some user issues when a user checks an item in their cart, click's the 'In Cart' button, thinking that they go back to the 'Cart Contents', then get upset because they've added another item to their cart.
The .zip file contains 2 files, button_in_cart.gif and english.php.
button_in_cart.gif is just the button which reads 'Add to Cart' rather than 'In Cart'
english.php has the following lines modified in to show the mouseover as 'Add to Cart' rather than 'In Cart'
define('IMAGE_BUTTON_IN_CART', 'Add to Cart');
define('ICON_CART', 'Add to Cart');
Thanks to Chris Fink for pointing out this user issue and button creation.