The last five major events were in 1315, 1470, 1630, 1725, and 1868, which have intervals of about 140 years (note that 2008 is 140 years from the major 1868 event).
If a major earthquake were to occur on the fault, damage would be catastrophic. More than 1.5 trillion U.S. dollars in property exists in the affected area, and more than 165 billion US dollars in damage would likely result if the 1868 quake were to recur.
The estimated probability of a major earthquake on the Hayward within the next thirty years was estimated at nearly 30 percent, compared to about 20 percent for the San Andreas Fault, which can have larger earthquakes but farther away from a significant portion of the urbanized parts of the Bay Area.
(various rips from Wikipedia)