I think the success of COD4 really took IW by surprise, they thought they were producing a yearly update to an established game franchise (just like Need For Speed), that they could instantly forget about once it was released. When in actuality they produced a gaming phenomenon.
If they had the balls and the brains to say "Sod it! Lets run with COD4 and see here it takes us" they could have produced monthly weapons, perks, game modes and maps and been left with a game that was iconic and probably still number one to this day. Unfortunately all the innovation went into the game rather than the business model.
Of course they are tied into producing COD games on a 2 year cycle, so unless they suddenly employ loads more staff we won't see much difference when MW2 comes out. I wonder if they have even considered investing in dedicated servers this time round?
MW2 might get 2 map packs this time, but at the end of the day IW will be focusing their time and money on COD8 once MW2 is released.