On the YosemiteNews.info forum, Mike Condron his the nail on the head:
I think there will be a ton of no-shows. There doesn't seem to be a way of getting your hands on an unused permit.


From the Yosemite NP Half Dome faq
Is there any benefit to canceling a permit if I know I won't use it?
The Half Dome permit is non-refundable. However, if you cancel a permit far enough in advance, someone else may be able to use it. Once a permit is cancelled, it becomes available again via www.recreation.gov or 877/444-6777.

If someone with a permit does not show up, can I wait at the base of the subdome and take their place?
No, you must have a valid permit in possession to show the ranger.