Kelthic, having the serial number is usually proof enough of ownership as they are unique to each machine/part. If he files a report with the police and they get possession of the machine/part somehow (raid or its turned in, etc.) then that should be all the proof he needs.
Promod, I'm not sure you have the means to track it down. If you had the IP address of when it was in your possession then its certainly changed by now. If you somehow have the IP address after it was stolen you can use one of many IP address lookup websites such as this one.http://whatismyipaddress.com/ip-lookup
Use the info to contact the abuse dept of whoever owns that IP address. (Comcast, Cox, SBC, etc.) If you don't have the IP address of where it is now then you are pretty much sunk. Only chance you have now is if you have some program on your laptop which tries to dial out and supplies the IP address. Running a DNS server or something like that.