it would be nice to have an official documentation about inter-version compatibility, there is some partial documentation here: http://www.zabbix.com/documentation/2.0/manual/appendix/compatibility but it would be nice to have an official chart including all the combinations of agent,proxy,server 1.8 and 2.0 like the one I tried to create and attached here, it would greatly help all the people trying to upgrade to 2.0