The asteroid was spotted on June 22nd by LINEAR, the Lincoln Near-Earth Research project. This is probably the biggest known asteroid to have whizzed past this close. Check it out:
Click the image to see the Astronomy Magazine article with many links.
The asteroid peaks brighter than magnitude 11.0 at the places where the closest approach is visible, and it's already about magnitude 12.5 — fairly easy to spot in an 8-inch telescope — by 14:30 UT, 2½ hours before closest approach. At that point it's visible from Southeast Asia, eastern China, and Japan, as well as Australia and points between.
The asteroid will be very hard to observe after its closest approach, since it's departing more or less toward the Sun. To observe the asteroid you will need a good telescope (the bigger the better), excellent charts and the know-how to use them, and ephemerides from the Jet Propulsion Laboratory. But just knowing it's out there is the cool-beans part!