This is from a couple years back already (pre-Avengers, since he still had a full head o' hair) but in light of yesterday's news that Joss will be the sole voice providing commentary on The Avengers DVD, it seems a timely discovery.
Not being a fan of Doctor Horrible where this song first appeared as a bonus item, or This American Life for that matter, I had never heard of this before. Whedonesque really let me down. Or else there must have been a day I didn't check their site (impossible!)
Either way it's worth a listen, and honestly makes me even more excited for the extra features of Avengers, though I could live without them, I swear. I just have to buy a Blu-Ray player first.