On OS X, there is not 64Bit fantom driver. So unless we work around the fantom driver which Lego refuses to update, we don't have any chance to access the NXT from 64Bit JVMs.
On Windows, we can use winusb to access the NXT. However, a JNI wrapper has not been written yet.
On Linux and other UNIXes which support libusb, everything is working with both 32Bit and 64Bit JVMs.