Jesus, see JBoss Cache docu in http://docs.jboss.org/jbosscache/3.2.1.GA/userguide_en/html_single/index.html
In addition to replication happening asynchronously, even serialization of contents for replication happens in a separate thread to allow the caller to return as quickly as possible. This setting controls the size of the serializer thread pool. Setting this to any value less than 1 means serialization does not happen asynchronously.
And as a side note, we've seen issues with asynchronous RPC marshalling in Infinispan, see http://community.jboss.org/docs/DOC-15725. This issue would affect JBoss Cache as well, so better keep asynchronous marshalling off unless you know what you're doing.