When you use the new official jdbc driver (mysql-connector-java-2.0.14-bin.jar) the classname is not org.gjt.mm.mysql.Driver. This was the name in the old mm.mysql driver.
if you are not using any security, dont worry about that last part. Where did you give your datasource name JNDI name?
maybe you need to start over with a fresh of then mysqlxx.xml descriptor?
Also see my info here http://www.jboss.org/modules/bb/index.html?module=bb&op=viewtopic&t=forums/ you are safe using the org.git.xxx for your mysql driver, it is present in the new jarfiles still. THough you can also start using the "jdbc.something.mysql.Driver" thingy too which is the new name.