Not sure if this will help anyone, but we've been troubled with our ESXi 4.1 U1 hosts disconnecting from vCenter during VM live migrations. I went through so many troubleshooting steps by myself and with VMware Support, from looking at DNS, to vCenter Server IP, to rebuilding my hosts from scratch (reinstall), and looking at the vCenter Database as the problem. All along, the problem was not any of these, because it turns out according to VMware Support the problem was caused by the SNMP Agent. There is no KB about this yet, nor has there been a fix. My VMware Support ticket number is 11066179605 if you want to reference it. Good luck.
