You can also integrate with a third-party shopping cart then load a small iframe to show "2 Items in the Cart." Not my favorite way to build a site, but it is very inexpensive to build, and for small vendors the site becomes entirely static. Even if you build it in Rails, you can use page-level caching so after the first hit it's served as a static html file.

Page level caching is one of the most underused features of rails. Most applications show anonymous users identical content, and can tolerate that content being cached briefly. Setting this up gives you slashdot protection as well as access to a last chance degraded mode if there's a chain of catastrophic failures further into the app stack.

Exactly, this is the general idea of what I do in the ecommerce app I mentioned.

I guess I should have been more specific about my strategy for "generated" content in an ecommerce environment. The traditional ecommerce elements that should be real-time are; such as carts, pricing, availability, etc. Everything else is generated on update via our [custom] inventory management software.

The closest analogy I can give is Varnish's Edge Side Includes. It's not exactly the same, but it's very similar.