Since most (all?) of the junk is metallic, how about an orbiting supermagnet?
Depends on the composition of the material and the strength of the magnet ;^).
http://en.wikipedia.org/wiki/Space_junk
http://www.orbitaldebris.jsc.nasa.gov/
(before the Chinese event)
Here you go:
"...If magnetically susceptible, the debris could fall in a few decades due to the drag of the Earth's magnetic field..."
My guess is a lot of that metal is aluminum and magnesium, which are minimally affected by magnets.
My idea is a huge ball of styrofoam that would sweep up objects then reenter after orbital decay, which would be rapid for a large low density object.
Not sure how that would work out!
Problems:
Low encounter rate.
Engineering a "capture density" for the styrofoam.
Getting it up there.