Dell just decided that they're the best and no one's gonna mess with them. So Apple can announce it's getting greener and HP might have more efficient PCs, but Dell is the leader, or will be, 'cause they say so.
Dell really has been doing some marvelous stuff in the last year. They're allowing customers to offset the lifetime carbon of their PCs (and even themselves) and they have developed an excellent recycling strategy and efficient desktop and laptop designs.
But it's not enough, Dell is asking all it's users to take the environment section of Idea Storm by storm. Idea Storm is basically Dell's suggestion box, and while news there has mostly swirled around Linux since it's creation, the environment is likely a more important issue to Dell customers.
So go check out IdeaStorm's Environment section. I've just submitted an idea myself, asking Dell to create a Long Life Laptop, that will function well for five to ten years, instead of the average three.