take the iphone to an AT&T Store or Apple Store, and they should be able to disable the password some how and figure out the mobile number. From there, they should be able to find the owner.
Do it quick before the owner deactivates the account. AT&T can locate the owner via the number printed on the SIM card. Then they can get the phone to it's owner.
Good job on doing the right thing, most people would not be as lucky as to have someone as honest as yourself find their iPhone.
Yeah, my g/f lost her purse in NYC with the iPhone in it. Within 2 hours they figured out how to turn off the phone and that was that. Nice to see someone doing the right thing.