I have run into this same issue in our landscape:
SAP EP 7.3
SAP BO3 & BO4 (we are in process of migrating from 3x-4)
With BO3x, you can create a BO template based iView, but you will challenged for your BO3x credentials (uid & pw); even if system properties & alias are set for SAP logon ticket.
For BO4x, the SSO will work ONLY IF YOU ARE LOGGED INTO BO FIRST! If you log into SAP EP first, and then click a BO iView, you will be challenged for uid/pw. But if you are already logged in to SAP BP 4.x, your iView report will be displayed. Please see attached blog for reference.
http://wiki.scn.sap.com/wiki/display/BOBJ/BI4+Integration+into+the+SAP+Entreprise+Portal+7.0.x