A PC Today magazine article advocates choosing laptops over desktops:

1.  Smaller form factor means lower impact on both the manufacturing and recycling end of the lifecycle.

2.  Battery life issues force a low energy consumption design: ~50% less than a desktop.

3.  Many laptops comply with European standards for the reduction of hazardous substances (RoHS) and recycling (WEEE).

Interesting stats from the article . . .

1.  230 million PCs sold in the US in 2006

2.  250 million computers landfilled in 2006

Listings of the "greenest" notebooks, netbooks, and tablets.