Hay I am also getting same problem.
I am doing integration with Alfresco 4.2.f and JBoss 6.2.0. I am facing "java.lang.ClassNotFoundException: org.gjt.mm.mysql.Driver". PFA my standalone.xml (<%JBoss_Home%>\standalone\configuration), module.xml (<%JBoss_Home%>\modules\com\mysql\main), mysql-connector-java-5.1.17-bin.jar (<%JBoss_Home%>\modules\com\mysql\main) and server.log. OS is Window XP. Let me know if I have missed any configuration. I have copied mysql-connector-java-5.1.17-bin.jar file into "<%JBoss_Home%>\standalone\lib" location. Also "<%JBoss_Home%>\modules\system\layers\base\org\gjt\mm\mysql\main" created and use module.xml and mysql-connector-java-5.1.17-bin.jar file here but issue still persisting.
Also alfresco tables created in mysql without any problem.
Regards,
Sudarshan