I think that's exactly the idea. If an annual nudge gets some folks to set up ipv6, then there is a steady increase in the size of the ipv6 network. For the most part, no one will have any reason to turn it off again the next day.
Ok, so I was reading about that in the FAQs, that website owners are supposed to turn off IPv6 after today in order to measure the "impact". Not sure what they mean by that.
When can we start an annual "turn off ipv4" day? http://bgp.he.net/ipv6-progress-report.cgi suggests that we'll have to wait a while.