It's not a cluster...
It runs a single OS image on all 256 processors (2048 cores) and the whole memory block. It's like a HUGE desktop... It doesn't need an OS instance for each server like a cluster would.
Single OS makes life VERY easy for administration and management. One memory block and all CPU's under one OS is a HUGE thing for some application that DON'T scale on clusters!
It's a standard x86 box running standard Linux. That's a huge thing! who wants to deal with Power or Itenium or Unix??? everyone now wants to use standard OS and off-the-shelf software and IO, and SGI can run STANDARD x86 64bit SUSE and RHEL and Windows on UV, and that's the way to go because these are simple, standard operating systems of the future.