From what I've read, the search for the culprit typically stops at the ISP...when they get their name-to-ip mapping.
So it's then up to the internet connection owner to prove that he wasn't downloading / uploading. And from what I've read, no one seems to be able make that argument properly.
The smart people know that thanks to a little thing called NAT, there is almost no way to track users in a situation like that, unless the router is question is keeping DHCP, MAC, and access logs...to which your standard Linksys isn't doing.
Oh and I found out about it when I was working on their PC; one of them mentioned something about the connection being down and not being able to download music. I was really happy to hear that...