This gist from Nicholas C. Zakas looks really handy:

This function determines if the browser is currently in a particular media media mode. Use the same media query as you would in CSS or on a <link> element.

I can see myself using it quite a bit. His usage examples look like this:

if (isMedia("screen and (max-width:800px)"){
    //do something for the screen

if (isMedia("all and (orientation:portrait)")){
    //react to portrait mode

I’m thinking it might be even more useful for me to detect !isMedia('something) and fallback to js stuff in those cases. Not sure yet. Still need to ponder.