Starting with Battle for Zendikar, Prerelease Events will no longer come with a "seeded" booster. Confirmed by Mark Rosewater on his blog, prerelease boxes will now come with six regular Battle for Zendikar booster packs and a promo rare, which could be any rare or mythic rare from the set.
Director of Magic R & D Aaron Forsythe made the announcement on Twitter and the reactions rolled in.
Confirmed by @maro254 today: Battle for Zendikar prerelease has no color choice/seed booster. Just 6 normal packs + a literal random promo.— Aaron Forsythe (@mtgaaron) July 28, 2015