Edit:
I found something: "As he lay dying, Vader ceased to be. Anakin Skywalker returned. He asked his son to remove the cumbersome, fearsome mask that had concealed his face for decades. His mask and life support removed, Anakin looked upon Luke for the first and last time. He then died, his body disappearing into the light side of the Force. Luke burned the dark armor that had encased Anakin's crippled body in a quiet funeral pyre on the forest moon of Endor that night."
[From the official starwars.com databank]
"According to Pablo's Q&A on Hyperspace, Anakin disappearing does come from Lucas, and he specifically requested that it be stated expicitly in the databank."
[From the theforce.net forum]
You might also want to check this forum thread on nightly.net (haha, that rhymed).
