Sorry, I forgot about posting the release notes (they are in the ZIP archive), thanks for reminding me. Here there are:
BTM - Bitronix's JTA transaction manager
version 1.2-rc1, January 14, 2008
* BTM-6 Support for JMS credentials
* BTM-5 pooling of prepared statements / statement cache
* BTM-3 Upgrade to latest version of SLF4J
* BTM-2 error not reported when XAResource.recover() call fails
This release mostly contains a few fixes, updates and feature requests. The big deal is with the new prepared statement cache. You can check the JIRA entry to know more about it until I update the documentation.