The designer of one of those entries, the cardboard-housed Recompute, was kind enough to contact us with some more detail about his computer, including a few specs. The fully functional PC houses all off-the-shelf components, including an Intel Core 2 Duo CPU, and 2GB of RAM.
The designer, Brenden Macaluso, also mentioned that there's been some concern over the safety of using cardboard -- but his reasoning with regards to the materials is that cardboard is more heat-resistant than many plastics, with a much higher fire and ignition point -- 258°C and 427°C, respectively -- where plastics begin to melt at about 120°C.
The designer also stressed that the material that Recompute is made out is just one facet of its sustainability: just as important in this design is the idea that less time, labor and parts go into the production of the unit, and that there will ultimately be far less to dispose of when the computer has reached the end of its life cycle. Check some more photos of Recompute after the break.