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 ATM/FR/Ethernet to electrical signals on the copper pair).
Since DSL works over a copper pair (phone lines), and you already know phone lines are not shared with your neighbors, there is no chance in intercepting your neighbors traffic over DSL, without someone physically splicing.
However, when ISP router is in plain bridge mode (i doubt anybody does this any longer, RBE so much more effective), there is possibility that the router floods packets for addresses it doesn't know, just like a switch does when it doesnt know where a certain MAC address is. This would broadcast that frame out across all the "virtual circuits". Most DSL modems would then also filter this, so unlikely you would still be able to observe it, unless you had control over the DSL modem/bridge itself.
Sadly a bunch of modems have the admin panel accesible from the WAN side, probably with the factory password or something the ISP sets to all same devices.
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!
