import Blender
from Blender import NMesh, Material, Window

editmode = Window.EditMode()    # are we in edit mode?  If so ...
if editmode: Window.EditMode(0) # leave edit mode before getting the mesh

object = Blender.Object.GetSelected()[0]
me=object.getData()
for f in me.faces:               # loop the list of faces
  print "original coords", f.uv
  f.uv = [(0,0),(0,1),(1,1),(1,0)]
  print "new coords", f.uv
me.update()                      # update the real mesh in Blender

if editmode: Window.EditMode(1)  # optional, just being nice