Captaincleanoff - if the hacker knows what he/she's doing they can't easily be traced, but for those of you free-riding on other people's networks, remember that they may be able to see information like your machine hostname - which may give away more than you expect. E.g. if their router is DHCP enabled, the DHCP station status page or log may show this in addition to the assigned address.
One other thing... I was back in the UK last week and without an internet connection when staying with one of my relatives. I found a neighbour's unencrypted network but it wasn't configured to automatically connect to their ISP (BTBroadband). Fortunately for me they hadn't set the router admin password so the connection issue was quickly fixed Fortunately for THEM I only wanted a quick internet connection, but I could have played hell with their router. Aside from MAC addresses and WEP keys, make sure you change the initial router password - many of them are left blank or set to 'admin' - if so, anyone getting onto your network only needs to visit http://192.168.0.1/1.1 or similar to take control.