Version 2 is here!
(see at the end of the description)

This is more or less a copy of the "Mini-shredder/Mini-filetador" from Takashi Utsumi

It is designed for wider blades:
Width: 18mm - Thickness: 0.5mm
The cutting edge length: 100mm
(the whole paralellogram is longer)

Links to the original:
http://www.utsumi.com.br/pet/English/filetador/index.html
http://www.utsumi.com.br/pet/English/dicas/minifiletadores_como_fazer.html
http://www.utsumi.com.br/pet/English/

Possible uses:
1) Weaving -- Make cool woven objects like those described on Utsumis pages.
2) Directly feed it into a hot end (to be designed) for printing (The melting point of bottle-PET is below 260°C edit: I sucessfully printed with 250°C)
http://blog.reprap.org/2009/03/drink-bottle-feedstock.html
3) Feed it into into a re-extruder to make 3mm filament for bowden printers.

After some testing with version 1 it is clear that PLA is definitely not suitable - the friction heat cuts right through the plastic after just a few 10m of ribbon. ABS might work but I think an incorporated L-Alu-Profie will be better.

Edit:
Version 2 is aviable !
This one uses an Alu-L-Profile for feasable wear resistance.
You'll either need an aluminum-L-profile of the following dimensions or change the *.scad parameters.
width = 25mm; height = 15mm; (length = 135mm)
To make the production of the alu part easier a drilling & sawing template part is included.

Building:

v2:

• print the parts with GRASP and TEMPLATE in it's names.
• (Do not print the part with ALU in its name its for future people with cheap metal printing capabilities.)
• Cut the alu-L-Profile cut to the right lengh.
• Put it into the inside of the template.
• Saw and drill all necessary holes and slits into it. (the slots have 20° slant angle.)
• Put a short M5 screw through the top hole of the alu-profile.
• Depending on the length of your blade put a longer M5 screw through one of the sequence of holes.
• Put a long M5 screws through the bottommost holes.
• Place the blade between the short and long screw cutting edge facing the slits.
• Screw it tight with two washers and nuts.
• Onto the long screws stick on the grasp part.
• Fixate it with washers and nuts (acorn nuts if aviable).

v1: (dysfunctional due to quick wear)
Due to Friction heat the PLA walls between the slots wear out very quickly.

• After printing drill out the viewing holes at the top and file off any internal mess.
• Put the blade in with the cutting edge facing to the entry side.
• Put a short M5 screw through the top hole facing inward & shift the blade against it.
• Screw down the top of the blade with a washer.
• Put a second short M5 screw throuh the longhole and the hole in the blade.
• insert a second M5 nut with a fitting small wrench from the bottom
• screw it down with an screwdriver from outside.

Using:

• The bottom end of the bottle has to be cut off with a conventional Stanley knife.
• The straighter the initial edge you cut is the better will be the PET ribbon.
• BE CAREFUL WITH KNIVES - I WONT BE RESPONSIBLE FOR ANY INJURIES
• PLASTIC EDGES CAN CUT YOU TOO
• With this tool: Never push always pull on the PET!
• Start with the widest channel. It's the easiest.
• Find the right tilt angle: Too steep and it stalls and tears; Too flat and you loose ribbon width consistency.
• Bottles without any embossed structutres are a lot easier to cut.
• Its use is a bit tricky! Be aware that It takes some practice to use it, don't give up right away.

PS: Be prepared to be called a clutterer when starting to collect PET bottles.