As Joe said. If you read their FAQ on their site, they answer this:
How will World IPv6 Day impact Internet users?
One of the goals of World IPv6 Day is to expose potential issues under controlled conditions and address them as soon as possible. The vast majority of users should be able to access services as usual, but in rare cases, misconfigured or misbehaving network equipment, particularly in home networks, may impair access to participating websites during the trial. Current estimates are that 0.05% of users may experience such problems, but participating organizations will be working together with operating system manufacturers, home router vendors and ISPs to minimize the number of users affected. Participants will also be working together to provide tools to detect problems and offer suggested fixes in advance of the trial."