Came across this last week after upgrading to RC9; I wasn't careful to ensure that my client libs for EJB3 were identical to those used by the server.
In my case:
The serializable proxy currently isn't specifying a serialVersionUID, and mixing-n-matching versions of the EJB3 JARs led to the error. This is documented in http://jira.jboss.com/jira/browse/EJBTHREE-402.
Try checking that your EJB3 JARs on your client's classpath are the same as being used by the deployer on the server?