My MacBook was stolen, but I know who has it. It is pword protected.?

they are connecting to an unsecured connection if they are even able to use it. Can they somehow override my pword and if so would I be able to see the MAC address on the unsecured network connection. I've reported it stolen to the police, but they said they can't get a search warrant for it. If anyone can help, it would be appreciated. it's a brand new MacBook running Leopard and I don't have MobileMe.