Looks like VMware has made a small but momentous change to the vmnic/vmhba numbering with the release of vSphere ESXi 5.5. In previous versions, the vmnic numbering was always determined by the the PCI ID. After adding a new ESXi Host to a Cluster that has been updated from ESXi 5.1 to ESXi 5.5 i noticed that the vmnic numbering was inconsistent although all host were identical. The newly installed host has a different vmnic to PCI ID assignment compared to the hosts that have been updated from ESXi 5.1:

To confirm that, i did a clean install of both versions on the same hardware:

VMware-VMvisor-Installer-5.5.0-1331820.x86_64.iso

VMware-VMvisor-Installer-5.1.0-799733.x86_64.iso

The result was the same. ESXi 5.1 has a different vmnic numbering compared to ESXi 5.5. After verifying that in the DCUI i discovered a possible reason behind that different numbering. It seems that the first priority for the numbering sequence is now the Slot Number: