Quoting from the IBM official announcement:
IBM today introduced a pair of ultra-powerful high-end machines, including the world’s most powerful server, the IBM System p5™ 595 – a 64-core speed demon capable of a record-shattering four million transactions per minute at a more affordable cost per transaction than HP’s flagship Integrity Superdome.
IBM attributed the huge leap in performance over industry competitors to the company’s revolutionary new Dual Stress processor technology, pioneered for ultrafast videogames and making its first appearance in System p5 machines.
The new systems leverage IBM’s leadership Virtualization Engine™ technology offerings to accommodate up to 10 virtual servers – or partitions – per processor core, enabling clients to consolidate multiple systems and distributed applications – even entire IT infrastructures – on a single box.
VM/core ratio is a value highly liable to hardware resources and workloads so it makes sense only related to a certain scenario, but provides a minimum term of comparison when evaluating different platforms.
VMware indicates an average VM/core ratio of 8 for its datacenter product, ESX Server, and 4 for Server and Workstation.
Microsoft never indicated an average VM/ratio on Virtual Server so no comparisons at all can be made.