Marketing talk about the "System of Systems" jargon
http://www.theregister.co.uk/Design/graphics/icons/comment/stop_32.png
IBM's Mainframe was made to run z/OS and that's the only system it run effectively... not just due to HW or OS reliability, but largely because all the IT staff surrounding it.
Let's face it, for just one mainframe box running some LPARs for critical workload processing, there are dozens of IT high qualified (and well paid!!) professionals working to keep it healthy. Since they don't have to support thousands of different machines most even now in details the source code from a legacy COBOL application which supports their company business. Within the Unix and Windows environments the situation is not even remotely comparable... a support team productivity is often measured in terms of "hundreds of machines per employee", thus making impossible to provide the same effort for supporting a critical system....
The integration of POWER and x86 servers is being marketed as it is part of the mainframe technically speaking, when in fact is nothing more than normal racks interconnected through Ethernet... The advantage of integrated management using the HMC is basically software, so besides a strategic reason I can't see why it couldn't be done with Blade Chassis sold separately and connected in the same way...
By the way, the idea of the RAIM memory sounded very interesting at first... humm, a RAID of memory... but then I remembered that even a simple 2 socket Intel x86 machines already provide Memory Mirror for some time, isn't that similar to RAID 1 as well?? I couldn't find further details, so maybe it is something like a RAID 5, but then I also wondered... is there a RAIM controller for that or the processor has to spend cycles managing that? Would that controller have cache!?? Well, nevermind...