Description of problem:
The logout request created by the SAML2LogoutHandler needs to have the format set on the NameID. Even though the spec says it is an optional attribute, 3rd party projects such as Shibboleth mandate it.
The NameID format can be adjusted by using teh "NAMEID_FORMAT" configuration option on the SAML2LogOutHandler in the picketlink.xml file:
<Handler class="org.picketlink.identity.federation.web.handlers.saml2.SAML2LogOutHandler">
<Option Key="NAMEID_FORMAT" Value="urn:oasis:names:tc:SAML:2.0:nameid-format:email"/>
</Handler>