If you choose to map the shib 'eppn' as the worpdress 'Username', then any permalink urls containing the %author% are at risk of breaking. The shibboleth plugin, unlike the standard wordpress user signup, does not produce a "sanitized" (http://codex.wordpress.org/Function_Reference/sanitize_title) version for the user_nicename field. Solution: add a filter (as suggested at http://wordpress.org/extend/plugins/shibboleth/faq/), I use:
add_filter( 'shibboleth_user_nicename', 'sanitize_title');