The "Promo Code" pack-in was included in the "initial run of the product" as a "special reward," going by the game's official site. Open to United Stated residents, the offer simply requires those that that got a "Promo Code" in their copy of Darksdiers to redeem the code on the PlayStation 3 Offer Page or Xbox 360 Offer Page by March 7, 2010.
Since Vigil's apocalyptic action-RPG Darksiders was only released on the two consoles, only the PlayStation 3 and Xbox 360 versions of Volition's destructive action game Guerrilla are up for grabs. As a bonus, you'll also get a digital Darksiders art book.
Seems from reading cheapassgamer.com that that code is only in SOME Darksider games. So if you haven't gotten it by now you have to have a bit of luck to get a game that includes it.
If I knew I could get both games, I might order Darksiders. But I don't. So I"ll get around to Darksider's eventually, but I don't trust this enough to push me over the edge. Got to work down my pile a bit more before adding Darksider's to the stack when it gets even cheaper.