IP = ID?? How can this be possible?

A man in WNY was arrested and sentenced to a year in jail over an email with the sole piece of evidence being an IP address? (- and a wireless IP address at that?! -)

How can they determine from an IP address who in the house or on a network is actually on the computer?
They can determine who the IP address belongs to (or belonged to at the time of the crime) by getting a subpoena to have the ISP turn over the data linking the IP address to an ISP account. They cannot, of course, determine from that data alone who was actually using the computer. If someone was actually sentenced to 1 year in prison based on an incriminating e-mail, I'm sure the prosecutor had more on the plaintiff than just the IP connection (a confession, more than likely...).