I've noticed that (for my use case anyway), this plugin doesn't work in IE. As per the Picturefill docs on supporting IE, I've added a line that, as well as adding a noscript fallback, adds a fallback image without a
data-media attribute for IE 8 and older. You can see it here:
Would be great if you could add it, or maybe advise if I'm doing it wrong.
(By the way, it would be great if you could fork and do pull requests on plugins in the WordPress plugin directory - I think Github has spoilt me!)