Saw on PlanetUnreal (by way of DNF World) that Sgt Hulka's Bootcamp recently posted some more IRC interview stuff that offered some insights from 3DRealms' George Broussard on why Duke Nukem Forever, their upcoming Unreal-engine shooter, will not offer support for cooperative multiplayer play, along with more on the general reason this seems to be a common theme in current shooter development, which may not make coop fans feel any better, but perhaps explains why it isn't a conspiracy against them either. Here's the quote (thanks again Sgt Hulka's Bootcamp):

Ghostinmy: Georgb3dr--- yeah is duke foreva going to have co-op support :-)

GeorgB3DR: DNF coop: Probably not due to the single player level structures. Once you get beyond simple levels like Doom or Duke 3D and have cut off points and scripted stuff you can't do co-op well anymore