I did not know about this, but someone pointed this out last week and I figured it was worth blogging about as this feature can potentially impact your design. (Think HA admission control policies and resource management)
When you create a VM in an Org vDC which is defined as a Reservation Pool you can actually manually set the shares per type of resource (memory and CPU) and also set a reservation and even a limit if and when needed. Pretty cool, but as you can imagine also very complex at some point to figure out to what it should be set.