Have you ever found someone else's misplaced phone? It happened to me once several years ago at a large outdoor concert. It was a nice handset but without the use of today's recovery / tracking apps, I had no way of finding its owner.
Fortunately, the phone rang as we were exiting the venue and I was able to meet back up with its rightful owner. Such a coincidence is rare and considering that most smartphones these days are (or should be) password protected, doing the right thing with a lost phone can be difficult.
In the "lock" section of the latest version of the app (or the web interface), you'll now see the option to add a recovery message to the lock screen as well as the ability to add a phone number for the finder to call to return the device.
The message you add is up to you - perhaps offer a reward or just try to win them over with kindness and guilt them into returning the phone.
Of course, there's no way to guarantee that whoever finds your lost phone would be the type to actually return it but hey, you might as well give yourself a fighting chance, right?