I'm not really getting the appeal here. 45 wimply processors in 4U doesn't sound that great, given that anyone can get 16 conventional sockets in the same space - 512 cores, 64 ddr3/1600 channels!
maybe the story here is the backplane, which hasn't seemed to get much coverage. does it provide something other than 2x Gb and power?
I believe HP has done something denser than VME (45 processor boards in a 4U) but probably without its mil/aero ruggedness (and cost) and more aligned with PC server standards such as PCI Express and Ethernet interconnects
The cartridge form factor here is smaller than anything I have seen from the VME and CompactPCI folks that fits into a muscular server-targeted chassis.
The VMEs of the world might sharpen their pencils over this one. Maybe they could break into the mainstream server world with another chassis.