It's "Jizz in my pants" - by The Lonely Island, the group includes Andy Samberg and they've made over 30 digital shorts for SNL.
Favorites include: Jizz in my pants, Giraffe and Dick in a Box.
According to their blog linked above, The Lonely Island's debut album "Incredibad" hits stores 2/10/2009.
** Update: New video for "Best look in the World"
"Iran So Far" - Cameos by Adam Levine and Jake Gyllenhal