What JAR files are you including in your app? (Also, what type of apps? WAR? EAR?) This type of class cast exception often happens when a class provided by JBoss AS (DataSource in this case) is also located within a JAR packaged with the app.
If that doesn't help, you could add the "-verbose:class" JVM option to JAVA_OPTS in run.bat or run.comf. This option causes the JVM to print the location of the JAR file for each class loaded. This can help pinpoint when classes are loaded from unexpected JARs.