Fine work on the documentation for this one. The documentation for Membership.MinRequiredNonAlphanumericCharacters suggests that the attribute in the <add> element when defining a provider should be minAlphanumericCharacters, or alternatively minRequiredNonAlphanumericCharacters, depending on which bit of the page you read.
In fact neither of these work, and you should use minRequiredNonalphanumericCharacters instead.
