Okay, Roger Friedman on this showbiz411 site has an "exclusive" that supposedly confirms Bill Corden as the Baker. He also mentions Johnny as the Wolf, Meryl Streep as a "rumor" for the Witch, and some speculation about the Baker's Wife.
From Suedlib at the Zone: "I was in attendance the other night at an event with Stephen Sondheim and he, at least, confirmed Johnny's participation in Into the Woods as the wolf. He said rehearsals were to begin this summer with filming to begin this fall in London as mentioned here earlier. He seemed quite pleased to make this announcement"