Shibboleth has a long and tortured history with logout as a concept, and nothing's really changed in that regard, but after many conversations, we have provided a very limited form of logout as part of V2.4.0 (due Q1 2013) that supports the basic use case of terminating a client session with the IdP. This is a very difficult feature to explain to a user, and should only be deployed after careful consideration.
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