@javax.ws.rs.core.Context javax.ws.rs.core.SecurityContext sc;

@GET
@Path("test")
@Produces(MediaType.TEXT_PLAIN)
public String test() throws Exception {    
  Principal principal = sc.getUserPrincipal();    
  MyEJBLocal myEjb = (MyEJBLocal) new InitialContext().lookup("java:comp/env/ejb/MyEJBLocal");
  return myEjb.test();    
}

@Local
public interface MyEJBLocal {
  public String test() throws Exception;
}  

@Stateless
public class MyEJB implements MyEJBLocal {
  @Resource javax.ejb.SessionContext ctx;

  public String test() throws Exception {
    java.security.Principal principal = ctx.getCallerPrincipal();
    ....
  }
}