If you are running Virtual Server and your Virtual Machine is having trouble obtaining an IP Address from your DHCP server, it could be because you have port security turned on on your switch.
AFAIK, when Virtual sever is directly connected to the host network or if it is configured to perform NAT, it configures the physical network adapter to run in Promiscuous mode. More importantly, as described here, outbound packets from the Virtual machine will contain the MAC address of the Virtual machine.
So, you now have a NIC advertising two MAC addresses (the host os and the VM). When you have port security turned on, the switch will ignore packets from the second MAC address.
To fix this, go to your switch and either turn off port security for the port used by your physical network adapter or increase the max number of mac addesses to secure on the port.
I did not find any info about this on Google which lead me to post this. If any one has more information, please post here.