Darksiders 2 will be automatically upgraded to the Limited Edition version, which is just the standard version and a packed in DLC voucher.
THQ's press release describes the DLC in question, "Argul's Tomb," as "the game's first single-player downloadable content pack." It also states that the Limited Edition voucher can be used "once [the DLC] becomes available," implying that it may not be released on day-one.
Furthermore, a $99 pre-order-only Collector's Edition of the game has also been announced, which includes a DLC voucher, life-sized Death Mask with stand, exclusive in-game armor and scythe set with "enhanced stats," a "digital version" of the soundtrack, and a hardcover art book. We can't wait; this is exactly what we need to finish our Casey Jones costume.