So I was browsing around NASA's photo archive looking for a nice high-res image of Io and I came across this little movie of Jupiter-shine on Io. It's a time-lapse movie in which you can see the Io moving so that the sun is on its far side — it starts off behind and to the left which is why you can see a bright crescent at first. As the sun moves behind Io from the camera's point of view, Jupiter, which is behind the camera becomes more fully illuminated. The sunlight bouncing off Jupiter in turn more brightly illuminates Io's surface which is why the night side of Io gets darker as the crescent of Io gets smaller. Pretty cool, eh?
You can read more about planetshine in this post, which also talks about how much light you'd get from the sun at different distances.
Another cool little movie on the same site is this one, which shows some interesting happenings on Io's dark side. The bright spots away from the edges are volcanoes, whereas the two blue glows on the edges are similar to aurorae on Earth. Except those aren't the poles, but rather at the equator where charged particles are colliding with the tenuous wisps of Io's atmosphere. The thin atmosphere itself is only existent because it's constantly being replenished by volcanic gases. Nifty.