According to a report by The Hollywood Reporter and as posted by Slash Film, Viggo Mortensen is "A Person Of Interest" to play the part of General Zod, a well know enemy of Superman's family in Krypton, and a prominent figure in the Man of Steel's rogues gallery. Played in the original Superman movies by Terence Stamp, Zod is a megalomaniac Kryptonian with the same powers as Supes and a thirst for power and domination. Seeing as how Zack Snyder is working on the reboot along with Christopher Nolan, plus another rumor that Kevin Costner may be up for the role of Jonathan Kent, Viggo Mortensen would be an excellent choice to play the role of Zod should he be chosen and he accepts the role. I mean, just imagine him pitching that excellent charismatic voice he used as Aragorn, and twisting it into an evil and maniacal one that says "KNEEL BEFORE ZOD!", bwahahahaha!XD
For now, it's speculation at this point, but if you want more info on the report, visit the source posts at THR and Slash Film!
The Superman Reboot, starring Henry Cavill in the titular role, is slated for a December 2012 release!