I've tried it both ways; adding our vCenter server that has multiple clusters and standalone hosts under it (combination of ESXi 4.0 and 4.1), and adding just one of the ESXi4.1 "problem" servers. It's weird because, looking at one 6-host cluster specifically, only two of the hosts show up in Monitor. All hosts, from what I can tell are identical...same version (ESXi4.1), DNS/domain/gateway info, same time (NTP) setup, all reachable via hostname/FQDN/IP, and i can connect to all via VIC. I've installed Monitor on a different server just to be sure it wasn't something with my install and the exact same hosts had issues. If I add the host (standalone), as I refresh the Monitor client, it toggles between "not responding" (greyed out in GUI and says "Collector state: stopped") and responding. When it says "not responding" the error message says "Unable to update infrastructure tree. Can't delete entity with ID 534 because it's a host node".