Ideally you'll use hardware IP load balancing round robin yes. It IS possible to use DNS (multiple MX records) but this is best used as a secondary failover since it doesn't balance the load at all. JBAS can't do anything for port 25, 110 or 143 type traffic since these are stateful protocols more or less. Meaning the session is good so long as the connection is good.
Clustering is envisioned to work in JBMS 1.0 with a shared/replicated DB ala RAQ and friends (depending on your service guarantee, that is mainly for reliability). There will probably be a follow on 1.5 which will allow different different DB configurations.
We have some enhancements that need to be done for POP with regards to clustering that have been pushed past the M5 release, but aren't critical (they just mean that feasibly users could hold on to locks longer than they should accross a cluster).