A few people have made comments about WP e-Commerce not running in WordPress 2.8 but its just wrong wrong wrong. You do however need to be using the latest Beta 4 release or the official Release Candidate being launched tonight - as usual everything is available here:
http://www.instinct.co.nz/e-commerce
You can see it in action here:
http://apps.instinct.co.nz/wp-2.8.1/products-page/
wp-e-commerce
