I think we also have the same issue.
We have a 6.7 vCenter and running some Client-OS in virtual machine. These machines are restarted nightly and only some virtual machines lost there network connectivity after the reboot.
These virtual machine will boot via PXE-Boot and as the virtual machine will not receive any ip-address from dhcp, the boot-process failed.
After removing the virtual NIC, re-add a completely new one and assign the same mac-address than before, the virtual machine will boot from network again :-)
I will look forward to create a VMware request - but if anybody have a new solution, please provide it, thanks.