On newer browsers, you can match @media states like “(min-width: 45em)” using javascript, then apply conditional code. I only found this out last night. Today, I came across http://wicky.nillia.ms/enquire.js/. It not only offers some additional functionality beyond the vanilla javascript. It can be made backwards compatible even with IE6.

One of the best use cases is only loading certain content (via AJAX) if the screen size warrants it, e.g. a sidebar or a higher resolution image.