The book will be an expansion of the line for both heroes; currently, Wonder Woman only headlines one title (although she appears in Justice League monthly and around the DC Universe). Superman has four titles in which his name appears prior to this--including the newly-launched Superman Unchained and the upcoming Batman/Superman. As with Batman/Superman, the Superman/Wonder Woman title will likely be scrutinized as indicative of whose name has more drawing power at comic book retailers, since Superman gets top billing here but not with Batman.
IGN, who have the announcement exclusively and provided the preview art at left, say that the new, ongoing title will "serve to explore the budding relationship between the DCU's most powerful heroes."
Of course, if DC Comics is going to double down on Superman and Wonder Woman's relationship, what is it that Gail Simone is doing with Lois Lane...?