A friend of mine build the same thing for Spotify called mutify, but Spotify released an update within a couple of weeks just making the advertisements pause whenever your audio is muted. The Spotify client will also pause the ad if the audio level in Spotify is below ~30%. Hulu will have a harder time though since it's web based (right?). Nice hack anyway. :)
Hmm. Interesting. I have to wonder if you could do similar for Pandora and NPR (although NPR can send me all the ads they want and I'll even pay them, as I love it so much)
So, it's a proxy that mutes the system whenever it detects 'adload' in the URL, and unmutes when it detects 'resume'. It's a clever trick, but unfortunately it's rather easy to ruin, and if you leave it up during other browsing you will probably get false positives.