Correct me if I'm wrong, but from my understanding:
Deployment of (session) EJBs as Web services is a J2EE 1.4 feature (Specifically, a feature of JSRs109/921 and EJB2.1 which are are part of J2EE1.4), and Weblogic 8.1 is a J2EE1.3 container. So, direct exposure of a session EJB is not possible.
The list of standards supported by WLAS8.1 is here.
WLAS9 is to be a full J2EE 1.4 container, but when I tried to use the beta version recently, it was _very_ beta - not realistically usable, even for a university project (which was my use). I've had some success deploying session beans as Web services on JBoss4.0.2, mind.
Of course, it would be possible to build a Web service that delegated all business logic to a session bean but I don't think that was your point.