From the NPS rock dude geologist:
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On the whole, 2007 was a relatively quiet year in terms of rockfall in Yosemite. Although the total number of documented rockfalls in 2007 was about the same as in 2006, the total volume of rockfall in 2007 was substantially less. This is probably at least partly due to the fact that the 2006-2007 winter was one of the driest on record. Rockfall activity in 2007 began at 0:28 on January 15, with a small rockfall from Castle Cliffs north of Yosemite Village. The largest rockfall of the year fell in early spring from below the Northwest Face of Half Dome, above Mirror Lake. This rockfall was about 290 cubic meters in size. A much smaller rockfall at 2:53 on June 9 proved to be more serious, as it sent small rock fragments into the southwestern portion of Curry Village; a number of hard-sided cabins were subsequently closed for the summer. 2007 ended with storm-triggered rockfalls from Middle Brother in late December, leading to temporary closure of Northside Drive. Other areas experiencing rockfalls in 2007 included Taft Point, Lower Cathedral, Half Dome, and El Capitan. In all, there were 35 documented rockfalls in 2007, with an approximate cumulative volume of 640 cubic meters (1910 tons). No injuries from naturally occurring rockfall were reported. It is very likely that there were additional rockfalls in 2007, but these events either were not witnessed or went unreported.
If you witness a rockfall of any size, please contact park geologist Greg Stock at (209) 379-1420, or at greg_stock <at> nps.gov. Predicting rockfalls is not yet possible, but understanding the events that do happen is an important step toward this goal.