Rainchill,
Did you ever find a resolution to this issue?
I'm having a similar issue with my hosts. I have narrowed the trigger event down to any reconfiguration on the 1000v switch, it may also happen on any virtual switch I just can't test it that way. I thought it was a vcenter issue at first since it always happened after a reboot/restart. This is happening on 6 hosts out of 44. Always the same hosts. I have other hosts on this exact version without issues. Vcenter 4.1U3 and ESXi 4.1 502767
At first the only thing I could do to get management back was to reboot the host. I have also discovered that I can disable the management network and then re-enable it and get the host to come back into vcenter.
This is only a management issue for me. The VMs are still running and I can remote console into them via RDP or SSH. I can gracefully shut them down before rebooting the hosts.
Thanks