instead of using DefaultDS why don't you try using mySQLDB and the poolname is mySQLDS. hope it helped
As I've written in a response to an earlier discussion, I've experienced hanging mySql connection when the connection pools are being set up while running JBoss on Linux. Reason was that the pool tried to connect as 'email@example.com', whereas only 'msql@localhost' was defined in the user database in mySQL. If you'ære running Linux, try adding 'firstname.lastname@example.org'.
Also note that the attribute 'Blocking' seems to make the pool block e.g. when a connection can't be made. Setting the attribute to false probably reveals the error, e.g. a Nullpointer.