That's about the size of it. If you are connected, the device you are connected to knows this, and it does not then tell the world that you don't exist. Some have taken firewall appliances (stand-alone devices) and used these to send that 'the destination doesn't exist' message, but this is detectable rather easily in the case of the home user because that box is actually doing NAT or Port Forwarding, and thus the address of the device is the one being targeted - you just happen to be forwarding those packets to the "real" computer offering the service. Also, by studying the packet headers (flags, options, TTL, window size, sequence numbers, etc.), it's often possible to identify at the very least, the operating system, and version. There is a company in Europe that was used as a spam support site, and using traceroute I could see them blocking pings at their perimeter router, which seems to have been a Cylades. But using other common network tools (several of which are even available for windoze users), I could also see there was a Cisco 7000 router behind that and then their internal network which seemed to be a mix of W2k and 98SE.
Bottom line - if you know what to look for, and use the right tools to look for it, there is a lot of things that you can discover.
Old guy