Keep the phone charged and someone will call back. Or hack it, its easy, use jailbreak tools etc or this
http://www.youtube.com/watch?feature...&v=sVV9S17mZpw
not sure if it works since I don't have any iphones to try it on
Given how upset my friend was to lose her phone (mainly because she seems to have lost people's phone numbers) I think most people would be really happy to get it back so your efforts are not in vain.
Hope the owner hasn't had to fork out $$$ for a new phone already then miraculously gets it back. That'll be annoying
I don't know if iPhones do this, but on a samsung 'emergency number only' screen you can see the owner's ICE numbers (if they've set any) - does it have any ?
If it doesn't have a SIM lock you could check if numbers are stored on the SIM. SIM has some limited memory for number storing too. Not sure if the iPhone will transfer numbers to the SIM though, but it's worth a try.
If it has no SIM lock you could call somebody and see the number.