The idea of biometric authentication plays straight into the view that each user has one "true" identity underpinning multiple authorisations.  

 I recently noted in the thread on identities and keys that: [We need] identity frameworks (like the Microsoft developed Identity Metasystem aka Cardspace) that permit as many "identities" as there are contexts in which we assert ourselves.

We are in the midst (I hope!) of a shift to a new paradigm based on a plurality of identities. And I think I'm using the over-wrought "p word" here in its proper context. The current "singular identity" paradigm has had a deep and unhelpful influence over the way we think about all sorts of things, including smartcards, PKI, biometrics, the semantic debate over "authentication" versus "authorisation", and therefore the underlying architecture of many approaches to federation.