People who already own the game will not be left out as Capcom will offer this coop element as a paid download (no price has been divulged as of now). This rumor has still not been confirmed by Capcom but if this turns out to be true it would actually motivate me to finish this game (umm again *cough*).
Darn! I just can't seem to find the realease date of Dead Rising Platinum!! If anyone knows please let me know. 
