Bungie is currently auctioning off a cameo voice over in Halo 3 at EBay. The money which is generated by this auction will be supplied to Hollywood Arts, a homeless charity that reaches homeless young people through the arts including: video-gaming, music, movies, fashion, hair, makeup and drawing, providing them with artistic training and a chance to change how they see themselves; create a new community of mentors; and learn life and job-readiness skills to transition them off the streets into school or jobs.

One interesting part is a mention of Halo 4 in the auctions description which "confirms" a 4th game is planned even knowing Bungie revealed that Halo 3 would be the last in the series.