Old thread. But i had this issue after doing an update to U1 of 5.1.
Turned out my vmnics were renumbered, we have 2x 10GBe and 2x 1GBe. It swapped the numbers around. a quick edit of the /etc/esx.conf and properly renumbering the nics and a reboot fixed our issue.
just did a vi /etc/esx.conf and changed the number on each corresponding nic. Oddly enough this only happened on one server out of 10 identical hosts.