How can I contact the owner of a lost iPhone who has a password lock?
1) Find the phone number, then find the owner

I think the iPhone is locked but not the SIM card. If this is the case, pop the SIM card out. Put it in another phone. Find the number. Now, use the web to do a reverse lookup, find them, find their email or alternate phone, and call them.

2) Find someone who knows the owner

When you set the password, by default the "Voice Dial" is still active. Try holding down the home button for 4 seconds. Even when locked, you might be able to say "Call so-and-so". Try some random names. If someone picks up, explain the situation. They will probably have an alternate means of contacting the owner.

3) Turn the phone on and see if there are recent callers - then use any of these names to try voice calling as in 2 above.