Can you say "Network Sniffer?"

It would be a cakewalk to figure out who it was if you happen to check email. ISP_A can call up E-mail Provider_B and usually find out what account the person logged in with if they suspect something was up and with most ISP's all connections are logged.

Also the wireless router should be logging the MAC address of the card, which uniquely identifies your computer.

However tracking directly to you at the keyboard is a tougher feat, but by no means is it difficult to trace someone's activity on the network.