http://www.theregister.co.uk/2006/08/03/industry_drops_filesharing_case/

And she used exactly what I was going to use in my defence:

Marson argued that as a cheerleader teacher she had had hundreds of girls through her house, any one of whom could have used her computer. She also used a wireless internet network, meaning that people outside of her house could have used her internet connection. "She doesn't even know what a shared folder is," said Kouretchian.

The bold bit, not the cheerleader bit...

The excuse that someone else could have used her PC to download music wouldn't really stand as a defense because it could be implied that, unless the person had broken into her house, permission (and therefore liability) had been given. However with a wireless AP in use, it can be easily argued that someone could well have used the connection to do something illegal without the owner ever knowing about it. No amount of wiretapping, network sniffing, etc is going to be able to get historic records of the devices NAT tables so there's no way to prove guilt unless the hardware (PC) is analysed...

http://www.newsday.com/news/opinion/ny-wh-nsawiretapping,0,1906650.flash