Good blog entry from Jon Masamitsu about NUMA optimizations in Java 6 on Solaris. NUMA, essentially, and vastly simplified, is to access a region of memory that is physically closer to a processer on a multi-processor system. This way there's less latency when reading/writing to the general memory region. For Java, the optimizations are made primarily to the Eden (young generation) heap space, as well as assigning a thread to a particular CPU.
To enable the feature. it's a command-line param to the JVM at startup: -XX:+UseNUMA .