Yes, exciting ideas out there, I was close to back one of the many plastic grind/extrude projects...
But it really doesn't work for me, as I did not not have a place in the apartment to set something up like this.
The other issue is plastic quality; first off there are different material compositions, and then the extrusion has to be precice ands with consistent density. Of course the 1,73mm UP extruder is more picky then my Printrbot's 3mm extruder... ;-)

But some ABS has an ideal melting point of 230 degree Celsius, some 260 degree... If different scraps get ground and mixed together, I would imagine the print results would be pretty bad.

At 20 euro per Kilogram that's a lot of plastic you need to recycle in order to save a bit of money, but yes, from an enviromental point of view the base idea is great.
There are so many innovative ideas out there, I am sure we will see more people recycling soon.
Though I wonder how enviroment friendly it is on a household-scale to wash, grind and melt a few pounds of plastic compared to recycling on a large scale.
On the other hand, a lot of plastic gets burned as it is either in the wrong trash bin or just impossible to sort.
Some products consist of three diferent plastics pressed, sealed or injected together... We would need a law against that :-)
Recently I have seen a report of a laser scanning machine that will scan grounded plastic particles and recognize it's type and color, sorting these difficult plastic container hybrids.
This will help to recycle plastics in the future, and help avoid more and more trash.

One thing I would find even more interesting would be some sort of PLA desktop factory, able to produce plastic from common components (starch or something) and easily available chemicals or bacteria.
Who knows what ideas some people will come up with next.