Question

During a AuthnRequest, is there a case where the AudienceRestriction

<saml:AudienceRestriction>
  <saml:Audience>http://serviceprovider.com/</saml:Audience>
</saml:AudienceRestriction>

would be different from the Issuer in the AuthnRequest

   <?xml version="1.0" encoding="UTF-8"?>
  <saml2p:AuthnRequest 
       xmlns:saml2p="urn:oasis:names:tc:SAML:2.0:protocol" 
       xmlns:saml="urn:oasis:names:tc:SAML:2.0:assertion" 
       xmlns:samlp="urn:oasis:names:tc:SAML:2.0:protocol" 
       AssertionConsumerServiceURL="https://serviceprovider.com/acs/web/sso/receiveSamlAuthentication" 
      Destination="http://idp.net/idp/SSOPOST/metaAlias/realm2/IDP"
     ID="http://serviceprovider.com/acsdata/data/AcsConfiguration/821212" IssueInstant="2010-08-20T14:48:27.620Z" Version="2.0">
              <saml2:Issuer xmlns:saml2="urn:oasis:names:tc:SAML:2.0:assertion">http://serviceprovider.com/</saml2:Issuer>
....
   </saml2p:AuthnRequest>

Answer 1

The SAML 2.0 Web Browser SSO Profile (from saml-profiles-2.0-os.pdf:566/577):

The assertion(s) containing a bearer
subject confirmation MUST contain an
AudienceRestriction including the
service provider’s unique identifier
as an Audience

So, it seems as though it should always be the case that they are the same, at least to be conformant to the published profile.