The SGI Altix is probably the world's largest production SSI Linux machine.
It is based on the SGI Origin 3000 architecture, but uses Intel Itanium2 CPUs and runs a fairly stock RedHat. SGI have released all their changes to the kernel etc as OpenSource on 
http://oss.sgi.com.
It currently scales up to 64 CPUs in a SSI and SGI have plans for 128-way models later in the year. Each machine can also be a node in a cluster, using the high speed/low latency NUMALink interconnect, and also having Global Shared Memory access.
It is one very cool system.