Ok, I didn't get it to work with Drakos code or an updated jquery, so I contacted a friend (http://vardevs.com/) who made a quick fix.
He wanted to rewrite the whole code, but we settled with creating and changing ajax_sc.js to a big javascript instead.
So all I needed to do was to replace ajax_sc.js.
Hm, looks like there was to much code to write in a post, so I uploaded in Ajax Buy Now-archive:
http://addons.oscommerce.com/info/4105
Hopefully it works for you too.
See it working at Lilon.se