1.2.0.Beta2 is functionally complete, with the exception of the "unreliable link scenario" (http://jira.jboss.org/jira/browse/JBMESSAGING-676), whose development is still on-going on a parallel branch.
Beta2 includes full clustering, with load balancing and transparent failover. However, it is still a preview release, not all functional, stress and smoke tests pass, so do not attempt to use it in production. The messaging team is concentrating to release an almost-production-ready 1.2.0.CR1 (all tests will pass, but we will be waiting for the community feedback) shortly. The final 1.2.0.GA is expected to be out at the end of February 2007.
The Beta2 release ships with clustering examples (which also double as smoke tests) that demonstrate distributed destinations and transparent failover capabilities. For the transparent failover example, a vanilla JMS client creates a connection to the JMS provider and sends message over that connection. Separately, we kill the cluster node managing the connection (using System.halt(), to simulate an abrupt VM failure), but the event goes unnoticed by the client, which continues to send and receive messages over the same connection. Obviously, the example requires a cluster of at least two nodes in order to work.