Earlier quoted context omitted.
People forget this. You can make your wireless AP as secure as you want, but if we're plugged into the same node with our cable modems, you can just run a regular packet sniffer with ARP poisoning and see all the traffic to your neighbors. Not sure if that works with DSL connections or not.
Mostly not possible. DSL has multiple deployment modes PPP (over Ethernet or ATM), Bridged, and routed-bridge encapsulation (RBE). The upstream router at the ISP is usually connected to an ATM or Frame Relay link, where they create virtual circuits to the DSLAM for each customer/modem (DSLAM is the last "network" device between your DSL modem and the telco -- it's the thing doing the Analog/Digital conversion from AT…
You still need to known your neighbors' public ip address, but the problem may be significantly reduced: "hey want to check my cool app?" Boom!