JBoss5 code:
Context ctx = new InitialContext();
txFactory = (TransactionFactory) ctx.lookup(
DistributedTransactionManager.JNDI_NAME);

JBoss4 code:
Hashtable env = (Hashtable) NamingContextFactory.lastInitialContextEnv.get();
InitialContext ctx = new InitialContext(env);
factory = (UserTransactionSessionFactory) ctx.lookup("UserTransactionSessionFactory");