There is no default transaction setting dictated by the EJB Specification:
EJB 2.0 Specification, Section 17.5 The Deployer is responsible for ensuring that the methods of the deployed enterprise beans with container-managed transaction demarcation have been assigned a transaction attribute. If the transaction attributes have not been assigned previously by the Assembler, they must be assigned by the Deployer.
If you do not specify a transaction attribute for methods in an EJB then it's transactional behavior will depend on the particular EJB Container that you are using. In the case of WebLogic, if a transaction attribute is not specified then the EJB or method behaves as if it has a transactional setting of "Supports". JBoss, on the other hand, seems to default to "Required". For the sake of portability, you should always specify a transaction setting for your EJBs.