An update for this thread: Due to the different answers I was finding on the various discussion forums, I sent an email to Disney World's customer service (guest.mail@wdw.disneyonline.com) asking about the policy for pocket knives now that the metal detectors were installed. This past Friday (4/15/16) I received a call from Vicki at Walt Disney World customer service. She said that she was forwarded my email and had contacted the security department directly to get me an answer. The official response from Walt Disney World Security is that small folding pocket knives with a blade length of 3 inches or less are allowable under Disney World policy. Of course this does not change the official "no weapons" policy Disney World has in place, but small folding pocket knives with a blade length of 3 inches or less are not considered weapons by Disney World Security.
I did not ask about any other parks so I would not consider this information to be relevant to those parks, just Disney World.
A previous poster mentioned a security guard using his ID badge to measure a blade, I am guessing the ID badge is around 3 inches long and makes the policy easy to enforce without the need for a ruler. So there you have it, I was impressed Vicki took the time to call rather than respond with some vague email. Hope this helps anyone else with the same question.