The user parameters are broken on trunk. Here is the terminal out of of a local test on a machine.

ginger# fgrep foo /usr/local/etc/zabbix_agentd.conf
UserParameter=foo,/bin/echo woot
ginger# /usr/local/etc/rc.d/zabbix_agentd restart
Stopping zabbix_agentd.
Waiting for PIDS: 7713 7714 7715 7716 7717 7718.
Starting zabbix_agentd.
ginger# su - zabbix -c '/bin/echo woot'
woot
ginger# zabbix_agentd -t foo
foo [m|ZBX_NOTSUPPORTED]
ginger# _

Zabbix 1.4.6 gives me:

foo [t|woot]