Since I only want to monitor one host, I set StartAgents=2 in zabbix_agent.conf, the I started the agent (sudo /etc/init.d/zabbix-agent start). agentd started OK and showed up with 2 processes in htop.
The problem is that the frontend did not get any data, and the server apparently could not connect to the agent. Please see screenshot at http://hotimg26.fotki.com/p/a/209_233/17_7/zabbix_Server_notavailable.png
zabbix_server.log contained "Get value from agent failed. Error: ZBX_TCP_READ() failed [Interrupted system call]"
After setting StartAgents=5 and restarting the agent, I could get monitoring data successfully.