I found the problem. Someone else had tried to install the Alfresco content management system (at least I think this was the culprit). As part of its installation, it appears that this application installed the MySQL Connector/J version 5.1.5 into Glassfish's domains/domain1/lib/ext directory, *alongside* my 5.1.10 jar. The net result was two jar files with various mostly identical MySQL JDBC driver classes. Who knows what was happening.
Thanks anyway,
Laird
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