Sometimes you need to render a line item in the cart differently than the rest.  

A sample usage case would be to prevent the quanity of a cart item to be changed.

A built in feature of the cart rendering, is the ability to render each product type with a different template. 
You can define the template via layout xml, using the "addItemRender" action within the 'checkout.cart' block

However, it is impractical to go an create a whole new product type. This article show another, very very easy way

Recently I noticed a long(ish) SQL query happening during checkout
The same calculation could be seen in the cart.

On the site, the SQL could take up to 2ms to process, thus adding unwanted delay to the end user experience

My investigations reveal a potential bug in how magento deals with quotes where no address has yet been selected.