I have modified following file wp-content/plugins/wp-e-commerce/wpsc-theme/wpsc-products_page.php
but I don't see any changes on the category page.
Also I removed following folder:
wp-content/uploads/wpsc/ and still I see only default styling of wpsc-products_page.php
It looks like a bug. I have the same problem on test shop. Try to use this tool first http://imm.io/vaRe and try to remove saved cache.