Calculated Items with SNMP items is calculated only first time, then error.
In zabbix_server.log
32279:20100914:182043.621 Item [Terminal:test] error: Format error or unsupported operator. Exp: [8733236 8562288]
32279:20100914:182043.622 Parameter [Terminal:test] is not supported, old status [0]

Calculated Items
last("ciscoMemoryPoolFree")+last("ciscoMemoryPoolLargestFree")

ciscoMemoryPoolFree SNMPv2 OID .1.3.6.1.4.1.9.9.48.1.1.1.6.1
ciscoMemoryPoolLargestFree SNMPv2 OID .1.3.6.1.4.1.9.9.48.1.1.1.7.1
With Zabbix_agent items it is work well.

See
http://www.zabbix.com/forum/showthread.php?t=19181