I'll put it this way:
I work for an ISP and do abuse trouble tickets....We get a letter from Sony, Fox etc saying "someone at xx IP has downloaded 'insert movie here'....Please tell them to remove it"...In turn we send a DMCA Notice to you based off the IP provided saying "Hey we didn't tell them who you were but they know you downloaded this and please remove it from uploading"....Basically they're ALL people using Pirate Bay or even worse, Limewire and other p2p programs like it. Invite-Only torrent sites or newsgroups are the way to go and you won't get caught. Technically after 3 DMCA notices an ISP is supposed to terminate your service. We don't really do that but I can't speak to Comcast or others. And yeah as stated before you can use the excuse "I have an unsecured wireless network" but that only really works the first time.
Just lettin ya know how things work from the inside out