Zabbix 1.8 for debian lenny can be installed from backports. There are available zabbix-server, zabbix-agent and zabbix-frontend-php in repository. This example shows how you can install zabbix-agent.
There is change in zabbix-agent version 1.8, disk free space is returned in bytes. See ‘Free disk space bug‘ in zabbix forum. This can cause problems when you want to use same template for debian lenny system with 1.8 zabbix-agents from backports and some still not upgraded etch systems. Backported 1.8 zabbix-agent for etch can be helpfull – you can find it here.