Not that I know of, he was tweeting about it : )
But, say you have a function that colours widgets. Everyone with JS enabled gets coloured widgets.
Now say you only wanted users with the regular "screen" media to get coloured widgets, while those with a media query of max-width: 320px don't run that function and don't get the funky coloured widgets (they get whatever you have when that function doesn't run).
So this isMedia() function is only checking to see IF a stylesheet was loaded, which would only happen if a user agent understands the query to that media type (the UA must know what max-width: 320px means or they won't load that stylesheet anyway). So you can decide if different functions run.