If just a memory unit is required, the financial burden on those without disk drives won't be so hard ... but honestly, is anyone using their Xbox 360 without some storage method? We've contacted publisher Activision (and Jamie Kennedy, of course) for more information.
Update: Infinity Ward community relations manager Robert Bowling told MyGamerCard that the icon should be disregarded. "The game can still be played by users with and without Storage devices (HDD / Memory Card)," he said, adding that you won't be able to save without some storage. We're still awaiting word from Activision.