[Lucid:]

My shortest "song" is One Last "Whoo-hoo!" For The Pullman, on Sufjan Steven's Chicago album. Though it's really just a couple chords and then a shout of "Woo-hoo!" It's 6 seconds long.
I have a 9 second song, Here Come The 123s, by They Might Be Giants. It's an intro track that just sings, "They Might be Giants, Here Come The 123s!" so I don't know if it counts either. (There's a similar 11-second song for their "Here Come The ABCs" album)

I do have four 10-second tracks that I would start to consider songs. Horrible Theme from Dr. Horrible's Sing-Along Blog, Phil by The Bird and the Bee, Horn Intro by Modest Mouse, and When I Go Out by Five Iron Frenzy.
Then I have a 13 second track, Dig Your Grave, from the same Modest Mouse album.
At this short length, it's hard to say whether an instrumental interlude is a song or not. They are in some ways, but some are just bridges from one song to another.

The shortest totally-absolutely-unquestioned song I've got, because most of the previous were instrumental interludes and/or jokes, is Fluffy by Harry and the Potters. Since the whole band is kindof a joke, the fact that the song is a total joke doesn't negate the fact that it's a song.

Once you get up to 20-second songs, I have LOTS of tracks of that length. between having a lot of soundtracks, alternative, ambient, and folk music, there's a lot of short interludes and for-fun songs in my iTunes. There's a Relient K song called Crayons Can Melt On Us For All I Care, and the lyrics go, "I just wasted 10 seconds of your life." and that's the end of the song. XD