Encrypting sensitive information in JBoss configuration
There is a wiki document out there: http://www.jboss.org/community/docs/DOC-9703 that describes this.
for the spring part, you can use your own extension of spring's PropertyPlaceholderConfigurer with the String convertPropertyValue(String originalValue) overridden. As the javadoc for the method mentions it (actually in superclass PropertyResourceConfigurer):
Convert the given property value from the properties source to the value that should be applied.
The default implementation simply returns the original value. Can be overridden in subclasses, for example to detect encrypted values and decrypt them accordingly.
This means you can configure your datasource with ${encoded.value} in the spring xml file, and decode the value before injecting the decoded value into the datasource.