Updated WP eCommerce on a client site on Friday. When we did it 500'd the site. Had to disable all plugins and manually re-activate to identify the issue. It was WP eCommerce - conflicting with itself. (Like a diabetic in a candy store.)
Here's an excerpt to point you in the right direction:
http://content.screencast.com/users/AlphaAlec/folders/Jing/media/fde0ba80-2441-4d1d-806a-949e13b46d09/00000676.png
Thanks!
Craig | Forge & Smith Creative
[email protected]