What version of ESXi are you using?
SNMP (specifically SNMP "get" which actually fetches meaningful information) is not supported on ESXi 3. The information you are seeing for your ESXi nodes is coming from the ESX API.
Here's more info from VMWare: http://www.vmware.com/files/pdf/vmware_esxi_management_wp.pdf
Hope this helps.
VMware ESXi 4 doesn't support the OID for Volumes, Memory, CPU use, nor NIC use.
It does support NIC names and status, OS name and version, and probable a few other things.
To enable SNMP use:
Set Community string: vicfg-snmp.pl --server <HostNameOrIP> -c <CommunityString>
Start SNMP service: vicfg-snmp.pl --server <HostNameOrIP> -E