Background: We were testing NIC failover with our new HP DL380 G7. In this case we had vmnic0 active and vmnic1 standby. When we would physically disconnect vmnic0, we would failover as expected, but when we would plug vmnic0 back in, HA kept pushing the VM to another host. Long story short, we found out turning Port Fast on on our Cisco switch failover to work seamlessly. We turned port fast on for all ports except the uplinks to our core switch.
Question: Is there any case where Port Fast can have a negative affect?
Portfast simply disabled STP participation for the port. It's a recommended practice by Cisco to configure ESX facing ports (including vSwitch and Nexus 1000v uplinks) with Port Fast as there's no spanning tree running below ESX.
Without Portfast your ports will go through the Blocking- Listening-Learning-Forwarding transition states which normally takes approx 50sec under default configurations.
Another recommended practice is to enable BPDUFilter on the ESX facing ports. Just an extra precaution to disable the ports (and protect your upstream switches) should the port see any Bridge PDUs (behavior of typical STP).