Quoting scoop from post #36687 , the flash posted before this, it's "Shounen Heart by Home Made Kazoku".
The vocals almost sounded like Shingo2 (Shing02? sp?) to me. If you like this song and you haven't already heard these albums you may like them. Nujabes "Metaphorical Music" and Tsutchie "Thanks for Listening" both of Samurai Champloo soundtrack notoriety.