Check out following doc
http://www.vmware.com/files/pdf/vmware_esxi_management_wp.pdf
From Page 9:
"ESXi ships with an SNMP management agent different from
the one that runs in the service console of ESX 3. Currently, the
ESXi SNMP agent supports only SNMP traps, not gets. SNMP
write operations are not supported by ESXi. This agent is off by
default. To use this agent, you must enable the SNMP service,
specify at least one community, and configure a trap destina-
tion using the remote command line command vicfg-
snmp."