One problem with 64-bit Java is the amount of Memory it occupies. As of 2011 we feel that all application profiles we have encountered do not require 64-bit Java as they can not take advantage of using more than than 1.6GB of memory. In many cases it is better to scale horizontally (running multiple JVMs than ) as opposed to using one giant JVM.
That is not to say not to user 64-bit servers. In fact we are moving to use entirely 64-bit systems these days most importantly to take advantage of all the memories (most of our servers use over 4 GB).
...