When I first installed the WP-Ecommerce plugin on a website this morning and tried to go to the products page, I got this error:

Warning: array_merge() [function.array-merge]: Argument #1 is not an array in /public_html/wp-content/plugins/wp-e-commerce/wpsc-core/wpsc-functions.php on line 622

Thankfully, it is actually very easy to fix this error. All you need to do is add (array) to the correct line of code in the plugin’s functions.php file.

Find this line in your wp-e-commerce/wpsc-core/wpsc-functions.php file:

$args = array_merge($wp_query->query, array('posts_per_page' => get_option('wpsc_products_per_page')));

And change it to this:

$args = array_merge((array)$wp_query->query, array('posts_per_page' => get_option('wpsc_products_per_page')));

That did the trick for me and hopefully will work for you too. Let me know if you have any questions or run into issues with this.