I've been working on writing an agent in Ruby for some special network level monitoring I hope to share soon. In the meantime I've posted some example code in the Wiki for writing a 1.1.x agent. It's pretty straightforward, but hopefully it will help others.
http://www.zabbix.com/wiki/doku.php?...xagentprotocol
http://www.zabbix.com/wiki/doku.php?...xagentprotocol