Bethesda could easily have made another ElderScrolls game after Oblivion and maybe 2 if they followed the CoD formula. Had they just done some graphic improvements a few tweaks and pumped out mostly an improved Oblivion in a Different setting Skyrim could easily be The 6th or even 7th installment in the series. However Bethesda builds each game with from the ground up brand new and different FAR different from the other. The Elderscrolls cannot work on the same formula in a different setting, CoD can or at least for now still sells. I mean I like CoD games and they could do a VERY good and revolutionary shooter if Activision allowed them to have say 3 years to make one but $$$ make that franchise go round and if people buy and love a game a year from CoD they will make it. RPG fans in general and especially die hard TES fans would never go for that approach which is why they have no problem waiting 4-5 years for a new game from Bethesda.
CoD out sells every game, cross platform it is likely that MW3 will outsell any game to date. However the effect of that on Skyrim sales will be minimal, Skyrim will in the long scheme of things very well. The elderscrolls is a very popular game franchise now. I remember when I played everquest 1 for 6 months( vomit inducing to think about it now) And I'd mention Morrowind to people as THE game I was looking forward to even though it was still in developement with only some concept art and rumors about it and they'd have no clue what that was and even when I'd explain it was the the sequel to Daggerfall only 3 out of 10 MAYBE even knew that game. Now days everyone almost knows of Morrowind and especially Oblivion so Skyrim will be on most gamers to get lists. It will not do CoD sales but not even Halo any PS3 exclusive or even GTA for that matter can beat CoD sales. The date will stay the same I think and Skyrim regardless will sell just fine and will have a life that will be full years after people have stopped playing MW3.