I have a friend who is in the Marines who lost his leg in an IED attack while serving in Iraq.
We were talking one day and mentioned MW2 and perhaps new things that might be included and the topic of, "What if they have like an IED thing for killstreaks/perk?" came up. He told me that he wouldn't like to see that in the game at all, reason being is that I guess he doesn't want to have something that is taking our and other countries soldiers lives to be depicted so nonchalant in a video game (especially because he has experienced that first hand).
I see that as a reasonable explanation why he and perhaps other real-life soldiers/veterans would not like to see that in a game and might be offensive to some players/critics.
Modern Warfare would get terrible publicity from tight-assed news sources like CNN. Its not worth it, and honestly would be completely pointless. Unless the game comes with vehicles in multiplayer, which I dont think it will.
Besides an IED doesent have any shape or design, its an improvised explosive device. This could be anything from old artillery shells set to explode to combustible materials packed with shards of metal.
You must realize that guns and bullets are everywhere not just in war. IEDs are too current and are a cowardly tactic-yet effective unfortunately. You can prevent being shot much more easier than being blownup by a remote device that you can make it look like litter on the side of the road.
People have lost their limbs because of grenades ... take those out too? Like other people have mentioned here it is a unfortunate part of warfare and has no reason not to be included/excluded for any other reason than it works/dont work (gameplay wise)