Yes, mod_jk can function as a load balancer; see http://wiki.jboss.org/wiki/en/UsingMod_jk1.2WithJBoss
You can use more than one apache/mod_jk box, with each sending requests to the jboss servers. Only downside to this is when a mod_jk instance makes its load balancing decisions it only knows about the request it has forwarded to jboss; doesn't know about requests sent by the other mod_jk instances. So it's calculation could be a bit off. Probably not a big deal in most cases. Obvious upside is as you said: apache/mo_jk is no longer a single point of failure.