EA has confirmed to CVG that it plans to release a separate product that features just the creature creation part of Spore.
Presumably it'll arrive before Spore in September, because with the separate product you'll be able to create creatures using the extremely flexible toolset designed by Maxis and then port them into the full game.
EA, however, was unable to give us a concrete release date. We hope it's free too.
Spore's the latest sandbox-'em-up from Will Wright and focuses on evolution. We won't babble on, but instead point you in the direction of this preview published yesterday to get details.