Our new Sun Fire V20z just arrived, and whilst looking around for the (GPL) source code for the Linux firmware of the onboard PowerPC based managment processor (which can be found by following the download link here), I realised it is just a re-badged Newisys box.
Further investigation lead to an interesting white paper on Horus, a large scale SMP chipset being developed by Newisys (the paper is a bit rough and doesn't appear to have been editied heavily, but worth a read). 4 Opteron processors connect to a Horus chip, which then link to form a larger cache coherent SMP system. Presumably they put four to a chip since four processors is about the sweet spot for the Opteron cache coherency protocol (cHT). Looks like it will scale up to 8-Quads, or 32 processors, talking over Infiniband. Supports dynamic partitioning too. Will be an interesting challenger and worth watching.
To save you looking up Horus he is the Egyptian God of the sun, with the head of a hawk.