Speaking with GameSpy during E3, Will Wright confirmed that Spore will be available "on all platforms," including cell phones - and presumably Wii. While Wright did not specifically name-drop "Wii," we must assume that "all platforms" includes Wii. In addition, Wright, by his own admission, is a bit of a Nintendo fanboy: "I'm rooting for Nintendo," he told GameSpy. Despite this statement, Wright was critical of the Wiimote, observing "the way the Wii controller works depends a lot on how close you are to your TV."
Interestingly enough, Wright currently favors the DS as his platform of choice. Any chance Spore will end up on Nintendo's handheld? [Update:Spore has been confirmed for the DS. Thanks, Sense.]
Spore is tentatively set to debut on the PC early next year (April/March 2007).