You might want to look at substruct, an RoR open source ecommerce application. See how they handle the shopping cart, or just use their app as a base for yours.
-Larry
Thanks for the plug, but that’s not the URL…that actually goes to some ASP site throwing an error. 
http://dev.subimage.com/projects/substruct - for project information, download, install instructions.
and the demo site…
http://substruct.subimage.com http://substruct.subimage.com/admin (admin / admin is the login)
Wow, that is a cool find. Looks like a great start on a basic e-commerce site.
The Blog would be better if it had commenting capabilities and syndication. And paypal for the checkout...but those both seem like very straightforward things to add on.
Joe
Actually it’s not based on the code in the book…(was designed before the book), but there are some similarities. Glad you’re enjoying it.