I guess your question is where to bind mongrel and the answer is to the
local ip since the external ip is not bound to you box by any mean,
instead,
the traffic is routed to you instance belong to that public ip(som magic
happens with the headers).
Here is the definition for public/private ips in ec2
EC2 Private IP Address: The internal RFC 1918 address of an
instance
that is only routable within the EC2 Cloud. Network traffic
originating
outside the EC2 network cannot route to this IP, and must use the
Public IP
or Elastic IP Address mapped to the instance.
EC2 Public IP Address: Internet routable IP address assigned by the
system for all instances. Traffic routed to the Public IP is
translated via
1:1 Network Address Translation (NAT) and forwarded to the Private IP
address of an instance. The mapping of a Public IP to Private IP of
an
instance is the default launch configuration for all instance types.
Public
IP Addresses are no longer usable upon instance termination.