Dont think your setup supports two phase commit (unless you are using a custom XA wrapper). Change to use local datasource for your mySQL instead of xa.
For what I can understand, my jboss installation is already configured for local datasource: mysql-service.xml has the following tags in it:
<module-option name = "managedConnectionFactoryName">
Is that what you mean ? Where else should I intervent ?
Found the solution: substituted mysql.jar wih a more recent version taken from mysql-connector-java-3.0.11 and everithing went to place.
Formerly I was using a very old mmjdbc version.