Yosemite has decided to make the hiking permit system apply everyday the cables are up starting in Spring of 2011. Previously they were only required for weekend hiking.
In 2011, free permits are available up to about four months in advance only through the National Recreation Reservation Service (NRRS).
As before, backpackers with an appropriate wilderness permit can receive a Half Dome permit when they pick up their wilderness permit with no additional reservation required. Rock climbers who reach the top of Half Dome without entering the subdome area can descend on the Half Dome Trail without a permit.
I haven’t been up Half Dome since they implemented this ticket system but I have friends who purposely hiked it on a non weekend day to avoid the permit system. Have you gone up there during the permit time, did you notice a reduction in crowds? They are limiting the tickets to 400 a day (with 300 of those for day hikers). I wonder if the day of long lines balancing between the cables will be over, or if folks will still tend to bunch up anyway.
More information is available on the Yosemite National Park – Half Dome Permits page.