CP stands for "Cumulative patch".
We create CP releases for our paying customers (i.e. those which have bought a support contract), and they mainly contain bug fixes.
We don't distribute binaries for these publicly, but all the source is publicly available in http://anonsvn.jboss.org/repos/messaging/tags/
You can easily check out from svn, then create the distro (ant distro). Of course, it won't be supported unless you have bought a support contract.
We hope to put out a community distribution fairly soon which contains all the latest fixes, timing allowing. We're all so busy on JBM 2.0 now though.