Reservations to access the Cables on Yosemite's Half Dome will be available on April 1, 2010 at 7:00am (PT) / 10:00am (ET).

Beginning Thursday April 1, reservations to climb the Half Dome Cables in Yosemite National Park will go on sale for all dates in July and August 2010. Reservations are required for anyone ascending the cable route on all Fridays, Saturdays, Sundays, and on all Federal holidays May 21 through October 11, 2010.

"The permits are ostensibly free, but include a $1.50 service fee per permit, so let's just say they cost $1.50 apiece. You can get up to four permits per reservation, and you can make reservations by calling (877) 444-6777 (518/885-3639 from outside the US and Canada) or by visiting the National Recreation Reservation Service website

If your expedition is larger than four people, you can get multiple reservations, but you'll have to go through the phone or internet process again for each one".
The body betrays and the weather conspires, hopefully, not on the same day.