With regard to the above, the short answer is no. Oracle still insist that every CPU processor/core/die (as appropriate!) is licensed since they specifically class ESX(i) as soft partitioning. But there is some good news.
Standard One and Standard editions are licensed on a per-socket basis (up to 2 or 4 respectively) but with a horrid caveat,
When licensing Oracle programs with Standard Edition One or Standard Edition in the product name, a processor is counted equivalent to a socket; however, in the case of multi-chip modules, each chip in the multi-chip module is counted as one occupied socket.
Although no proper definition of 'multi-chip modules' is given, the Intel Xeon 55xx series quad cores are on a single physical die, so one would presume that these cannot be multi-chip modules, unlike the older Core-2 based chips.
Net result - Standard One or Standard should be good to virtualise on dual 55xx systems like Dell R610's without further licensing issues - awaiting confirmation from Oracle on this....
Thought virtualization.info had a good point about Oracle and virtulization and how they can be serious if they still have issues with their licensing and supporting Virtual Machines.
www.phdvirtual.com, makers of esXpress