EDIT 11:54 am: I made some important corrections to this blog post based on helpful comments from "bcat" below.
I have a couple of updates on my post from yesterday. First, I received an email from minor planet tracker Bill Gray, who informed me that Chang'E 2 was seen by fellow asteroid observers at L2 (along with Herschel and Planck and some other flotsam) from time to time since its arrival there, but the last time it was noted to be in that position was on March 21. So there's some independent corroboration that it left L2 in April!
Second, user "Galactic Penguin" at the NASASpaceflight forum (who was the original person to post this news in the first place, evidently) has provided more information translated from the slides accompanying Ouyang Ziyuan's speech. Chang'E 2 departed the Sun-Earth L2 point on April 15; the Toutatis flyby is scheduled for January 6, 2013.
Furthermore, if Chang'E 2 lasts so long, it could encounter more asteroids beyond Toutatis.
I'm not sure if there's more information buried in the presentation about the encounter distances or velocities. Chang'E 2 has very high-resolution cameras designed for detailed mapping of the Moon, able to achieve 1-meter pixel resolution from a 100-kilometer orbit. That's 10-milliradian resolution, comparable to Cassini's Narrow-Angle Camera 6-milliradian resolution. But asteroids are small, fewer than 5 kilometers in diameter; in order to get a picture of Toutatis that is 100 pixels across with that camera, it would have to approach to a distance less than 5000 kilometers, which is very close indeed.
Apophis is only 300 meters across; a 100-pixel image of that would have to be taken from only 300 kilometers away.
Plus there is a HUGE difference between imaging the surface of the Moon from a circular orbit and targeting small bodies at high encounter velocities. It can be done -- both HiRISE on Mars Reconnaissance Orbiter and Mars Express HRSC have pulled off imaging of relatively fast-moving Phobos and Deimos from their Mars orbits, and Lunar Reconnaissance Orbiter managed to neatly frame the globe of Earth with the Moon's shadow falling on it recently -- but it is not at all easy to do. Can Chang'E 2 target accurately enough to pull off such imaging? Does it have (or can it be given) autonomous tracking capability? I don't know the answers to any of these questions. But I'm hoping they can indeed achieve some decent imaging of these little space rocks.
CORRECTION: In an earlier version of this update, I mentioned possible encounters with 12711 Tukmit in August 2018; 99942 Apophis between April and September 2020; and 175706 (1996 FG3) between August and December 2023. These would not be Chang'E 2; they would be future asteroid missions.