For technically minded FReepers, the following is provided:
IP's are tied to MAC addresses which are tied to specific network cards(NIC). Unless someone swaps out their NIC, it is easy to trace the time/date and verify the actual PC. Even if they do swap the NIC, most businesses have with data circuits have a static range of IP's they get assigned. If it was a DSL connection then the ISP can easily tell which phone line was used, and limit the scope of the search to a specific network within a house or business. If the connection was from a cable modem then the range is geographically assigned which means you can tell within a couple of blocks where it came from.
"IP's are tied to MAC addresses"
This is not always the case. If you have a static IP address, you are correct, but if you have a dynamic IP address, there may be no correlation to the MAC addres. The MAC address is, however, always directly tied to the network card.