From the Yosemite National Park Facebook page:
On Friday, June 22, at about 5 pm, a hiker slipped while descending the Half Dome cables. A fellow hiker dropped a handheld radio on the cables above him, and the hiker instinctively reached for the falling radio, losing his handhold on the cables. The hiker slid a short distance on his back away from the cables and was able to stop by grabbing a natural feature. The hiker waited precariously to be rescued by NPS rangers. During the time it took rangers to arrive on scene, other hikers on the cables dismantled a portion of the cables in order to help the fallen hiker to safety.
Due to the time of day the accident occured, the cables were closed for the evening to allow Yosemite trail crew time to assess and reassemble the cables. On Saturday morning, June 23, park rangers and trail crew finished reassembling and inspecting the cable system. There was no permanent damage to the cables. The Half Dome cables were reopened at 10 AM. The rescued hiker had no serious injuries, suffering only minor scrapes and cuts.
The accompanying picture:

Related to the accident, this was posted June 22:
The Half Dome cables are temporarily closed for emergency repairs. Trail crews are working to repair the cables and anticipate reopening at noon on Saturday, June 23, 2012. (This time is an estimate and is subject to change.)
If you have a permit for June 23, you can request a refund (but not a permit for another date).
You can call 209/372-0826 for more information and updates beginning at 8 am (on 6/23).