Paul Davey of Xtravirt published a VMware networking article a few weeks ago called vCenter MAC Address allocation and conflicts. The article describes the mechanism behind MAC address assignments in vCenter, and more specifically how conflicts are avoided:
When a vCenter server is installed a unique ID is generated. This ID is randomly generated and is in the range of 0 to 64. The ID gets used when generating MAC address and the UUIDS, or unique identifiers for virtual machines. You can see that if two vCenter servers had the same unique ID, a possibility exists that duplicate MAC addresses might get generated; cue packet loss, connectivity issues and your desk phone ringing a lot…
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Updated 3/14/11: I thought this might also be helpful for this article. VMware explains the automatic MAC address generation process as follows:
The VMware Universally Unique Identifier (UUID) generates MAC addresses that are checked for conflicts. The generated MAC addresses are created by using three parts: the VMware OUI, the SMBIOS UUID for the physical ESXi machine, and a hash based on the name of the entity that the MAC address is being generated for.
