If you have an android, and share your latitude location with friends, you could check them for your phones location. I honestly don't know if the iPhone has an equivalent functionality...
If you did not have software installed to help you locate it before before it was lost, your only option is to contact your cell phone provider. they may be able to provide information about any phone calls made since it has been lost.
Therer are some steps that you can take with your lost pfone and it starts with filing a police report. I know it wasn't stolen but there are benefits to having a police report filed. Next contact your cell phone company and have them notified that the phone has been lost and what the date was that it happened so that no new charges will be applied to your account. Then they can help you with tracking.
The links above are for you to look at and see if they would be any help to you , the 3gadget site would be a place to register your phones information in case it comes accross theier data base.