I've set 3 items for my server to get sensor data each 30 seconds but it Frequently changed to not supported
zabbix_server.log:
8042:20111025:103124.814 Enabling IPMI host [zproxy01]
8282:20111025:103134.311 IPMI Host [zproxy01]: first network error, wait for 15 seconds
8281:20111025:103141.545 Disabling IPMI host [zproxy01]
8130:20111025:103225.023 Enabling IPMI host [zabbix]
8290:20111025:103242.032 IPMI Host [zabbix]: first network error, wait for 15 seconds
8295:20111025:103336.089 IPMI Host [zabbix]: first network error, wait for 15 seconds
8291:20111025:103342.064 Disabling IPMI host [zabbix]
8123:20111025:103456.984 Enabling IPMI host [zabbix]
8102:20111025:103501.839 Enabling IPMI host [zproxy01]
8287:20111025:103539.066 IPMI Host [zproxy01]: first network error, wait for 15 seconds
8289:20111025:103540.094 IPMI Host [zproxy01]: another network error, wait for 15 secon

when it changed to not supported i had check with manual "ipmitool -U zabbix -H 192.168.XX.XX -I lanplus -L ADMINISTRATOR sensor"
i've got the following message:
Error in open session response message : insufficient resources for session

Error: Unable to establish IPMI v2 / RMCP+ session

also the result of "ipmitool -U zabbix -H 192.168.XX.XX -I lanplus -L ADMINISTRATOR session info all" maybe usefull (at the next possible time)
session handle : 1
slot count : 4
active sessions : 60
user id : 2
privilege level : ADMINISTRATOR
session type : IPMIv2/RMCP+
channel number : 0x02
console ip : 192.168.XX.XX
console mac : 00:00:00:00:00:00
console port : 4040

session handle : 2
slot count : 4
active sessions : 60
user id : 2
privilege level : ADMINISTRATOR
session type : IPMIv2/RMCP+
channel number : 0x02
console ip : 192.168.XX.XX
console mac : 00:00:00:00:00:00
console port : 59029

session handle : 0
slot count : 4
active sessions : 60

session handle : 4
slot count : 4
active sessions : 60
user id : 2
privilege level : ADMINISTRATOR
session type : IPMIv2/RMCP+
channel number : 0x02
console ip : 192.168.XX.XX
console mac : 00:00:00:00:00:00
console port : 25072