Salesforce with Shibboleth IdP

[Andrew Morgan]

Does anyone have a step-by-step guide for setting up Salesforce SAML v2 
with a Shibboleth IdP?

I am trying to use IdP-initiated SSO, but no matter what I try, Salesforce 
gives me an error:

   Login Error
   Your login attempt using single sign-on with an identity provider
   certificate has failed. Please contact your salesforce.com administrator
   for more information.


Google search hasn't turned up much content about this subject.

Thanks,
 	Andy