we have added support for Symmetric Multiprocessing but it will be available for our next release. not eta yet. but this is for dual-cores.
regarding clusters ... well we didn't thought about it. we tested dual-cores and they work fine with hd content, asterisk, mythtv, ...etc, there's no real need to have 10 servers to play some media, even hd.
we support only diskless media directors. the operating system is installed on core, not on md's hdd.
you can restrict access for users over orbiters.