To open a port to an EC2 instance, do:

• ec2-authorize -p <port number> <security group> (from any machine with the ec2 command-line tools)
• If the server has iptables: sudo /sbin/iptables -A INPUT -p tcp --dport <port number> -j ACCEPT (on the server)

Speaking of iptables, here’s:

• a nice, short and sweet tutorial from the Ubuntu folks.
• an exhaustive doc on iptables that starts with a recap of TCP/IP