Hey everyone, just wanted to get a quick post up in case anyone else ever runs into this issue. Promiscuous mode enabled on a virtual switch in vmware ESX (4.1 in my case) caused my snmp based monitors in ipMonitor to show as down for machines on that virtual switch. The error in ipMonitor was "The response from the remote device does not adhere to protocol specification". Setting promiscuous mode back to the default of 'reject' got my snmp monitors behaving again. I haven't put a whole lot of time into troubleshooting further, my workaround was to create another virtual switch on a separate nic port, and move the machine that required promiscuous mode onto that switch. I will be much more cautious with promiscuous mode in the future, and probably eventually open a case with VMware.
Thanks for sharing, dfsadmin!