Joystiq reports that Mortal Kombat may require players to register a one-time-use code in order to play the game online, in the same vein as the "online pass" system used by publishers EA and THQ.
The rumour stems from a tipster who forwarded an email to Joystiq that's apparently been sent to retailers. The email explicitly states that the game "includes a one-time-use registration code that gives players access to all online modes in the game. Players who do not have a code will get a free two-day trial of the online play and then be able to purchase the online modes for 800 Microsoft Points on Xbox LIVE Marketplace and (US)$9.99 on the PlayStation network.
However, this is still very much an unconfirmed rumour at this point, albeit an unsurprising one. Stay tuned for confirmation or correction as it lands.