Am I wrong here or is everyone missing the one point that can't be worked around. While the download may not be determined due to encryption, and the source may not be trackable due to anonymizers, to make the connection in the first place, the router assigns (albit temporarily in the case of DHCP) the IP address to a MAC address. This guarantees that the routing goes to the right place (assuming the IP address doesn't point to more than one address, but that is another issue that shows immediately to at least one person with that address). Therefore, with even the most basic logging, all sources, even wireless, has access to the MAC address of that interface.
THEORETICAL DISCUSSION POINT HERE - MAY OR MAY NOT BE FEASIBLE IN REALITY!
Once you have the MAC address, you can then follow MAC address connections and if you use wireless in your house regularly, you can determine where that MAC address connects most of the time. Even if not, you can narrow it to a specific location and frequent any location that seems to show a regular connection to the hotspot. Once you see that connection made, you found the person.
Is this time consuming - absolutely. Will it happen? Probably not. Is it possible in reality? This I do not know, as you would essentially have to gain access to each hotspot and search the history to track the MAC Address. This would require a HUGE amount of resources and probably a ton of cooperation that may or may not be available. Would someone attempt this for an illegal torrent, doubt it. For National Security, Weapons trafficking, or espionage? Not so doubtful. The real question here is can it happen vs. how likely is it to happen.