I wonder if the hardware at that scale isn't exactly stable - so to become a "select customer" comes with signing a gag-agreement not to publicly disclose stability problems - and I guess also requires the customer have suitably trained staff who can deal with any expected or unexpected problems - and software engineers who can write software that will fully utilise the processor's capabilities - much like how Sony took some PR flak for the PS3's Cell processor being difficult to program for given its atypical design.
