The Dutch police have an interesting approach to cell phone theft.

They give the network providers lists of IMEI numbers of stolen phones. In return, they get the phone numbers that are currently used on these stolen phones.

Then they (the police) start bombarding these numbers with SMS messages like "this is a stolen phone; return it to the police". The idea is that your friends will eventually want to know who is sending you these SMS messages all day ...

I have no idea how effective this approach is, though.