i think i've understood things, but i'd just like clarification on the use of <valid-connection-checker-class-name/> in a local-tx-datasource. am i correct in undestanding that this tag will allow jboss to reconnect to a database that's gone idle or has been restarted without the need for me to restart the JBoss AS server?
basically yes. If the check-result fails, the connection is closed and a new one is created. As developer/user you will not see any changes, because it happens all before you get the connection from the pool. See also http://community.jboss.org/wiki/ConfigDataSources