vSphere/vCenter 4.1 automatically sets vNIC (vritual NIC) type to be Flexible when a new VM is created, one is resulted after a P2V operation using VMware converter, or when one is upgraded to Virtual Hardware Version 7.
This may have adverse effects on a VM if drivers are not loaded correctly during or after booting up a VM. We noticed that a couple of our VMs were timing out in our environment and this was fixed by changing the adapter type e1000.
You can find more about VMware vNIC types here, http://kb.vmware.com/kb/1001805
For production environments, I recommend using e1000 even though your VM may eat up a few CPU cycles.