Zabbix is a great tool, I'm currently trying to put in place, the snmp trap management. I have followed the manual for the installation, but I have seen that the script snmptrap.sh is not complete.
For my version (1.6.2), there are some missing parameters as -z -k -o etc. You need to update the script. The recommended solution is certainly to put at the last line a "echo" and redirect the ouput to a file to see if the zabbix script is called by snmptrapd, and if the new command is well builded.
echo $ZABBIX_SENDER $ZABBIX_SERVER $ZABBIX_PORT $HOST $KEY "str" /tmp/snmpresult.txt
When you have the result in snmpresult.txt, you can modify and add the missing parameters.
$ZABBIX_SENDER -z $ZABBIX_SERVER -p $ZABBIX_PORT -s $HOST -k $KEY -o "$str"
The best solution came from Pierky (http://pierky.wordpress.com/2009/01/30/zabbix-snmp-traps-mapped-to-the-right-host/).