Family Guy's creator, Seth McFarlane, coud be one of the guests of the next and last Futurama DVD Movie, Into the Wild Green Yonder, according to a phone call of SciFiTV.com.au with David X. Cohen (Futurama Producer). Acording to the article, Seth will sing at the movie, and also, David said he has a "beautiful singing voice".
I don't know if this info is 100% real, so I think time will tell if this was true or not. Don't forget to visit the full SciFiTV article.