All the new evented server frameworks have big awesome benchmarks about number of open connections at once on a single server. But what about the other hardware in between the server and the client? Are all packets the same to the hardware (tcp,udp), or does an idle TCP connection take up more resources in any measurable way (other than packet size)
Any network device that does stateful packet inspection will incur overhead for each active TCP connection, idle or not. That means firewalls as well as anything that does many-to-one NAT (which is the usual NAT you are familiar with). That's why these devices have timeouts on idle TCP connections so they can eventually recover (memory) resources associated with them.
Any other type of network device, such as a router, incurs no overhead for idle TCP connections that happen to run through them.